jameslambnz wrote: … Is it as simple as updating my Apple ID on my old Mac and then syncing?
Pretty much.
If you sign on with the current ID, and you connect your Photos Library to iCloud, then all the pictures from the iMac will be copied to your iCloud Photos, and all the pictures at your iCloud Photos will be copied to the iMac Photos Library. Photos will do its best to avoid duplicates, and it won't remove any pictures just because they weren't on the iMac or iCloud.
Repeating: all the pictures at your iCloud Photos will be copied to the iMac Photos Library. Do you have enough room on the iMac for this? Be sure! If not, then you may need to copy your Photos Library to an external drive, make that the System Library, connect it to iCloud, and run from there.
Let us know how this goes and if you need more clarification…