Will it work to un-install Snow Leopard (booting from original disks) and then reinstall, then upgrade without erasing hard drive on iMac?

I have read on these support pages that you cannot get snow leopard freshly installed without erasing the whole whole drive since the original installation disks are older than the current updated version.

That is incorrect. You can install an older version of the same OS over itself. With Snow Leopard and later, unless you intentionally select to erase the drive, the OS removes and replaces all remnants of the current OS and leaves all of your third party apps, personal files and settings intact.

I have also read here that it is possible to get the system newly installed (though it's wise to have a back up of the rest of your files).

That is the correct information.

Is there an un-install function after booting from a different source? If I can get the system files off the main hard drive and then install Snow Leopard from my original disks, then go through the upgrades, and end up with the rest of my files intact, that would be wonderful.

No, there is no way to remove just the OS files from a drive. At least, not in any manner you could call easy. There are thousands of hidden files and folders related to UNIX. Many apps won't even run correctly, or at all, if they're not on the same drive you are starting up to. So trying to separate your third party apps from the drive the OS is on is not a good idea.

I have all the adobe design premium software on the system with their updates and would like to not spend hours and hours restoring it all.

If your end goal is to simply get the contents of the external drive onto a new internal drive in a bootable form, you can use Disk Utility to clone the drive, or use one of two excellent third party apps; SuperDuper! or Carbon Copy Cloner.

Yes, that is the correct procedure. Again, make a backup of the main drive first, even though it's having issues. You can then at least restore it back to that point if reinstalling the OS really botches something up.

Otherwise:

1) Boot to your Snow Leopard DVD and choose to install the OS to the drive SL is already on (along with your apps and user account). Make sure to check the install options to turn things on or off. By default, Rosetta is not installed. So make sure to turn that on if you need to use PowerPC apps. QuickTime 7 is also not installed by default in SL.

2) The system will automatically boot to the hard drive you installed SL to when it's done. Go into Software Update and apply all available updates. That will bring it back up to 10.6.8.

You're welcome, Kathy. 🙂