Upgrading past 10.6.8

Upgrading 10.6.8 to El Capitan (10.11) I want to upgrade my mid 2010 Mac Pro's 10.6.8 Snow Leopard to, say, 10.11 El Capitan (please don't ask why I haven't ever done this till now). According to Apple's "How to create a bootable installer for macOS" (HT201372) I can download the El Capitan package installer to install "Install OS X El Capitan.app" into 10.6.8's /Applications, and then use its createinstallmedia tool to create a bootable 10.11 installer on to a flash drive. Then I can use that installer to upgrade (a copy of) my 10.6.8 to 10.11. I haven't tried this as yet because my main concern is if I don't want to use the result how can I revert back to my 10.6.8? As I said I would do this to a copy of my 10.6.8 drive and ideally if I wanted revert I simply boot from the original 10.6.8. But what has me concerned is the line in that HT201372 doc (under Learn more near the end) which states: "A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model." Does that mean it changes my Mac Pro's firmware or it only accesses the appropriate firmware to add to the installer to allow it to boot on my machine? If installation changes some of my Mac Pro's firmware, presumably the stuff that affects the boot, do I assume that I cannot simply boot back my 10.6.8 once I've done one of these installs beyond 10.6.8? If this is the case then one way I can think of to revert the firmware is to install the original 10.6.4 installer CD to revert the firmware done by that install. And yes, I know you have to change the clocks back to about 2012 to do that. A much simpler way, if it is possible, is to use some terminal command with appropriate arguments to revert the firmware (nvram?). If so what would that command look like? Bottom line, I don't want to "upgrade" (?) beyond what I got until I am sure I have a backup path in case this doesn't all work out. If the firmware is not changed then all I need to to do is simply reboot from my original 10.6.8. Any help/suggestions would be appreciated.

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How to revert to a previous version of macOS I don't need to do this right now, but I'm just trying to understand the process. If someone updates their macOS (e.g. 10.14 to 10.15) and then decides they want to revert to the previous version (e.g. 10.15 to 10.14), what's the best way? (In these examples, I'm only looking at the option of erasing and reinstalling a copy of macOS; I appreciate there are other issues such as some applications may have already been updated to work with 10.15.) The key issue is this: having installed (in this example, 10.15), they need to erase their internal drive and install 10.14, but: (1) Booting up with Command (⌘) - R will install the latest macOS that was installed on the Mac (10.15). (2) Booting up with Option (⌥) - Command (⌘) – R will install the latest macOS that is compatible with your Mac (10.15) (3) Booting up with Shift - Option (⌥) - Command (⌘) - R will install the macOS that came with the Mac, or the version closest to it that is still available. Eg. 10.12. So none of these is the 10.14 wanted. So is the only way to achieve 10.14 in this example option three, and then upgrade from 10.12 to 10.14? (Obviously it the user has a Time Machine backup that was taken before the upgrade to 10.15 they could restore that instead after erasing the contents on the internal drive to remove the partitions on it.)

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iMac (Retina 5K, 27-inch, Late 2015) upgraded from High Sierra to Monterey 12.6, need help reverting back to High Sierra from Time Machine Greetings, I have a wonderful iMac (Retina 5K, 27-inch, Late 2015) that ran just fine using High Sierra, but I made the rather foolish decision (in hindsight, as this iMac is literally the oldest model capable of running 12.6) to upgrade to the latest OS - and for many reasons I don't need to bore you with, I have ultimately decided I made a very bad decision. To summarize, In very broad and generalized terms, 1) it's just far too taxing on this hardware, and 2) All the applications I rely on are no longer compatible and purchasing so much new software is not a viable option Now, if I had done this a couple days ago, I think it would be straightforward enough to just revert with a time machine backup, but regrettably, I was really giving 12.6 a thorough chance and weighing everything which took time, and dealing with a lot in my life at the same time greatly delayed that process even more, resulting in one of my big problems I am desperately hoping this wonderful community can tell me a solution for - I DO NOT KNOW what date it was when I installed 12.6. I think there is a VERY good chance I have backups going back far enough to when I was running High Sierra, I just can't find out how I can tell from the backups which ones are post-OS upgrade and which ones are pre-OS upgrade...or if there IS a way to find this information from the backups. This is hard for me, I worked for years in IT in Ivy League acadamia exclusively with Apple products, and was always my own expert, but a career change 15 years ago left me slowly losing all my expertise and suddenly I realize I know almost nothing anymore as I haven't kept up, and I suddenly realize I've fossilized, I'm almost useless and very lost! The little knowledge I still have is enough to add this - I DO have the system DVDs that came with this imac. I also have multiple hard drives and time machine only backs up the drive with the OS, so I am pretty sure I have a handle on how to keep all my data, including I can easily backup anything created after the time machine backup with High Sierra, between just copying it to one of the other drives and I have an offsite online backup service as well, I use Backblaze. So I think no issues there. I also feel pretty confident I could clean install an old OS, even prior to High Sierra, and upgrade back up to High Sierra if that's the only way, as long as there are complete system install disk images out there I can download, there always used to be I remember, so if that's my only option can that still be done? Thank you ALL so much for taking the time to read this, and I can't tell you how much this once-all knowing Mac guru appreciates you current all knowing and all powerful PROS giving me your time and expertise, I'm so serious, this is the first time I've realized I'm totally helpless and lost on this one and I have a new appreciation for getting old and being clueless, haha. I hope there's an easy answer and easy fix!!! Because this old iMac just cannot do what I need it to it's got to get downgraded. Thank you so so much for any help.

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