Release notes are your friend. No reason not to go to the latest code on all. You will need to be careful verifying by part number as well as model what are gen. 1 vs. gen.2 devices as the firmware image different and gen. 1 likely won't take the most recent code.
Making a big jump to the latest will probably just work, but rolling back may be tough. When upgrading, certain command syntax is modified automatically to allow for new features. For example, many commands now have the "ip" keyword to distinguish from "ipv6" which wasn't supported in earlier code. If you need to roll back, the commands with a leading "ip" may not be understood.
For consistency, you may want to only upgrade your gen. 2 devices to the latest code supported by gen. 1 so that everything looks and works the same unless you need specific features provided by later code on the gen. 2 devices only.