If you're already on 8.3(2) you've already gotten past the tricky bit - the new NAT syntax and access-list object use. There are some minor changes with identity NAT in going up to 8.4(3) as described here but that's about it as far as things to watch out for.
The TAC is quite helpful and it is a good idea to open a case proactively just to have them on hand to take a quick look at any issues that come up. The TAC security team deals with these upgrades every day and is very adept at zeroing in on the root cause of any issues you are having and setting things straight within in few minutes.
