You can do that (leave the old seal) but I've found it breaks down over time and then you get leaks that are pain to find and fix.
Fine a piece of pipe that fits through the wrist pretty snugly. (I've used 3" PVC with good success, if it's too small, wrap is with cardboard to get a little more diameter) Heat to peel off the old seal, then Acetone with a razor blade and scraper to remove the old glue. It's best if you can get it reasonable clean of the old glue.
I then put the new seal on the PVC pipe with the cuff rolled back, and butted up against the suit such that if I roll it onto the sleeve it's in position.
Prep is key to a good seal. I wipe the new seal with acetone, then scuff with 80 grit sandpaper, then another wipe down. Wipe down the suit side with Acetone as well. 2-3 coats of glue on the suit, at least 2 on the seal.
When the last coat is pretty dry, start rolling the seal up onto the suit. I try to start on one side, then do the part 180deg away, then work my way around. This prevents getting a big wrinkle if you are stretching the seal as you pin it down. If you get it a little wrong, hit it with the heat gun and you have a few seconds to soften it up and re-position it. Then hit it with a little roller to get a good set on the glue.
I like the DRIS dry suit glue. It's the best thing I've worked with so far. PB3000 is I think basically the same stuff and I think you can get that from lighthouse.
This guys videos are good -
https://www.youtube.com/watch?v=10gRSju2BW8 He also has a good one on neck seals that helped me learn to do it.
Replacing seals yourself is pretty straight forward, saves a little $, but I mainly like the convenience of being able to do it myself, not having to wait until a shop has time to deal with it.
Let me know if you have more questions.
Jake