Forum OpenACS Q&A: Re: Still using OACS 3.2.5...Upgrade? How?
I've upgraded old systems to the latest AOLserver without much problem. There may be a few minor things you need to change but nothing serious comes to mind.
Upgrading OpenACS itself would be a big job I imagine, which is one of the reasons we haven't done it on certain clients.
I can't comment on Postgres unfortunately.
hope this helps!