Thank you for that additional information.
I looked at your comparison chart, which was most helpful, and am a bit disheartened to find some of the tools I use to organize will be gone. I almost never use events so really don't care a lot about that. I do use folders and albums, so am happy they will be intact. However, within albums I use flags and stars along with the ability to sort ascending or descending as a primary means of organizing albums, moving the photos I am working on to the top of the page with 5 stars and sorting on descending.
For example, we had some whales feeding at a local cove recently. Photographing whales is challenging, so I came home with 350 photos with maybe a couple dozen worth paying attention to and only a half dozen worth the final cut. So easy to sort through those with stars and move the contenders to the top with stars and giving 5 stars to the final edits of the best ones.
Based on what I'm reading, not only will I be looking for a new organization strategy, but will also lose a decade's worth of organization when I migrate. Sounds as if those albums will be intact but the organization within the album will be gone. I have some albums with over 1000 photos. Looks as if I should create some new, smaller albums before migrating.
Also, I back-up to an external drive using time machine, on a regular basis, but am in the process of also putting my photo files separately on an external hard drive that can then be accessed by other computers via USB. I keep experimenting with strategies but have not yet found one I like. What is the easiest way to end up with a copy of each complete photo file (not just a thumbnail) on my external drive. Maybe export the library? Drag and drop the iPhoto library icon to the external drive? If I export the library does any organization go with the files or do I just have 25,000+ random photos on the external drive.
Sorry for for the rudimentary questions. I'm pretty good at the photo part of things but a lot more basic with the computer part. Learned photo with film and a darkroom so have been on a never ending learning curve with digital.
Thanks to everyone for all the help here.