Replace series 2 to a series 3 case?

While it might be possible, its a fair amount of work! You will need to fully strip down your watch and transfer all of the part over. Just doing this is risky! As you are likely to damage something. Given the price of a new Apple Watch 3 is so low and offers much more performance. I would just get a new watch!

Now, if you had one of the special Apple Watch Edition (real gold! Yellow Gold or Rose Gold; Black, Grey or White ceramic) and wanted to put in the newer guts into it then I could see that as being worth it! Some of these cases are $20k