Upgrading from 4.6 to 5.0 is more complicated than is typical for OpenACS, due mostly to the changes in the authentication system that allow for external authentication (i.e. LDAP).
Here's an entire thread on the process.
Regarding changes in form handling, what you're probably noticing is that the standard template for forms built by the template form engine has changed. Most of us find the new template more aesthetic and it also flags required fields. It shouldn't impact your application's functionality, though, just the standard look.
5.1 is coming out soon, we're in beta and it looks like our release candidate #1 will come out in a few days. You may want to practice upgrading from 4.6-5.0 then additionally explore upgrading to 5.1. If you do, I'd recommend you upgrade PG to 7.4.2. Might as well switch to AOLserver 4, too, if you've not already.
But of course only after a bunch of testing :) You'll be skipping major versions of both OpenACS and PG!
