Well, I thought I'd finally report what I did and then undid, as a result of all this discussion! BOTTOM LINE: DON'T UPGRADE AN EARLY 2009 MACBOOK TO OS X 10.10 YOSEMITE. But, I also learned a bunch of other information about how to go back to older Operating Systems, the use of Time Machine (which is better than most Apple Technicians know), the use of Migration Assistant, and the great, free software support from Apple's Genius Bars at their Apple local retail stores. Read on if you'd like to learn from me about those things:
I decided I wanted to upgrade from my 10.6 Snow Leopard system just to 10.8 Mountain Lion, because I suspected that OS X 10.10 Yosemite, and perhaps even 10.9 Mavericks, would be too much for my system resources. But, when I went to apple.com to upgrade, I could only go to 10.8 by ordering (and paying for) disks to be shipped to me. The only other listed option for my macbook serial number was upgrading to OS X 10.10 Yosemite. I was also having a printer problem which (at the time) I thought might be due to my outdated system, and didn't want to wait for disks to be shipped. And, since everything Apple Support had posted said that my early 2009 macbook could run OS X 10.10 as long as I had 4 GB of RAM, I went ahead and upgraded to OS X 10.10 Yosemite. That was a mistake. It was too much for my macbook's system resources, especially when running Office for Mac 2008 - Word and Excel started to run really slow. It would run good if I rebooted, but after a while of use, even if I closed various applications I had used, it would run really slow again - too slow to be efficient in my work. It seemed like either Yosemite or Office for Mac was not releasing system resources, even after closing things. A call to Apple Support - advisor suggested that I was right and things wouldn't improve even if I upgraded to Office for Mac 2011. He agreed that it was probably because of the greater resource needs of Yosemite. He suggested that the technicians at the Apple Genius bar at the Apple local retail stores could help me with getting back to my original 10.6 Operating System, or to 10.8, and made an appointment for me.
I went to the Apple Genius bar at the local Apple retail store - a great source of help! They helped me save Documents and iPhoto Library, and then cleared the hard drive and did a clean install of OS X 10.8 Mountain Lion. It seemed to work great. (And, one of the main benefits for upgrading from 10.6 to 10.8 is that the Safari version associated with 10.8 is still supported and gets security updates, etc., while Safari with 10.6 is not - something I thought important for working on the internet.)
On my way home, however, I remembered - "but what about all my 'macmail' emails"? I use my mac mail application with a lot of folders to store emails related to my work, like a filing cabinet. And, they are essential. Since I chose a POP configuration (instead of IMAP), the emails are no longer available on my various email servers. Another call to Apple Support - GOOD NEWS - the technician explained that I could use "Migration Assistant" to get all that back via my Time Machine backups. (Migration Assistant can be found under your Applications/Utilities folder in Finder.) Well, that technician said, however, that Migration Assistant will probably only be able to access the latest Time Machine backup that was created before I upgraded to Yosemite, since I was now at Mountain Lion - an OS earlier than Yosemite. That backup would have missed about the last week's emails, but that would still be pretty close. BUT, PLEASANT SURPRISE: Migration Assistant didn't ask for a specific backup date to use, just for me to tell it which backup drive to pull from, and God's blessing: it took everything from my very latest backup, which I had made while I was at OS X 10.10 Yosemite, right before reinstalling OS 10.8 Mountain Lion!!!! It not only got ALL my emails, it also restored ALL my documents, AND even my applications. For example, I was told that after restoring my system to 10.8 Mountain Lion, I would have to reinstall my Office for Mac 2008 from its system disks. But, not so with Migration Assistant - it was back again, also!!!
Subsequent to all this, I read an Apple Support article about Time Machine that said I could have just restored my complete system back to the way it was (when at OS X 10.6 Snow Leopard), just using Time Machine for that date. Of course, however, I would then not have my documents or emails I created or received in the week since upgrading to OS X 10.10 Yosemite. BUT, talking to another technician at the Genius Bar, there would be a work around for this as well. For documents, of course, you can just save your latest complete Documents folder to an external drive or large thumb drive. And, you can save your current iPhoto Library folder that way too. But, evidently, there's, likewise, a folder(s) that stores all your mac mail emails too (like your "iPhoto Library" folder) which could also be saved. Then, after doing the Time Machine restore to your previous operating system, you could then just restore these three saved folders, and you're back up and running with all your current data - under your previous operating system. I didn't try this, don't need to at this point, because Migration Assistant worked for me, but this is basically what the "Revert to a previous OX X version" apple support article said could be done - that is for going back to previous OS X version. The getting of your latest data restored, following that, is what I learned from the Apple Genius Bar technician.
ONE REMAINING ISSUE: My iPhoto Library. The one application I upgraded to work with Yosemite after upgrading to Yosemite, was iPhoto. Evidently, that changes the access to your iPhoto Library. So, two things had to happen: 1.) I had to go back to the Genius Bar to get them to install a version of iPhoto application that would work with my now OS X 10.8 Mountain Lion. That wasn't much trouble. We just sent the current iPhoto (Yosemite) version to the Trash box, and then installed the needed iPhoto application version. However, when we then tried to connect my latest iPhoto Library folder (which we had saved separately), it couldn't be accessed. The upgrade to iPhoto application (Yosemite version) had altered the iPhoto Library as well.
Next, we tried connecting my iPhoto Library folder from my Time Machine backups from a date before my upgrade of iPhoto application. But, this didn't work either. So, for now, we simply moved the photos in my latest iPhoto Library (from my Time Machine backups), to the iPhoto Library folder newly established by the install of the iPhoto application for my OS X 10.8 Mountain Lion system. This took a lot of time (48GB of photos/videos to move), but worked fine - except that it didn't have all the "Events" organization I had before. It simply had the events organized by date. I can reorganize that. The main thing is that I have all my photos/videos now, accessible from my new iPhoto application on my installed OS X 10.8 Mountain Lion system.
(I'm not convinced there isn't another way to get all of it back, including my events organization, but haven't had time to look into that more - and I'd probably want to do it with the aid of a Genius Bar technician, again.)
FINAL ADDITIONAL THOUGHT: Apple's products seem to operate BETTER than they promise.
1.) No Apple person I talked to - either on phone support or at two different Genius Bar visits (3 different technicians), thought that once I went back to a previous OS X version, after upgrading to Yosemite, that I'd be able to access any of the more recent Time Machine backups I did while at Yosemite - BUT, it did!!!
2.) AND, No apple person expected Migration Assistant to get back any of the data from my more recent Time Machine backups that I did while at Yosemite - BUT, it did!!!!!
I think the answer here is that Apple doesn't want to promise the results that could happen when you're going backwards in Operating Systems. But, I found it all to work rather well!!! I continue to be amazed at the quality and capabilities of Apple products.
Pete