Swapping HDDs w/ Different OSX Versions, Possible?
Since I am currently using a 20-inch, Mid 2007 iMac, I am somewhat familiar with upgrades. When I installed Snow Leopard, it ran as a 64 bit operating system. I have installed every version of OS X since Snow Leopard without any problems. I am currently running both Yosemite and El Capitan. I have run Vista 32 bit, Windows 7, 8, 8.1 and 10 64 bit, even though Apple only officially supports up to Windows 7 32 bit on my iMac. 

According to the site: OS X El Capitan - Technical Specifications, you can run El Capitan on your Mac. I assume then, you could also install an older version of OS X. Of course, finding one to download might be a problem.

Your question:


  Which OSX version, if any, can I upgrade to without jeopardizing my old setup? So if I swapped in my old hard-drive, Snow Leopard will be accessible and boot as if nothing happened?


Any OS X newer that Snow Leopard. You could even move your Snow Leopard to the new SSD and install another OS X version.

Your question


  Once EFI firmware is updated to work with the latest OSX, is it backwards compatible with older versions by means of swapping in an old HD? I also know OSX has some form of Keychain security, but is it used in the booting process? I will use the same local username/password.


People often return you computer's OS X to factory conditions. Whether this means using a DVD that came with your Mac or downloading from the internet, I do not think any Firmware upgrade would disable your ability to do this. In other words, Firmware upgrades should not prevent Snow Leopard from running on your Mac. 

Your question:


  Does signing in with Apple ID on Mavericks/Capitan/Yosemite lock the EFI to that specific OS installation? Like an anti-theft kind of deal? I know Apple is big about security...


No. I currently have two versions of OS X installed. Your Apple ID has nothing to do with the EFI.