Joe,
I have updated a few older OpenACS systems over the years. What is easiest is to come up with a planned staged upgrade between the multiple compatible versions from where you are to where you want to go.
So, the first step is to upgrade PostgreSQL to the newest version compatible with whatever OpenACS you have. And then upgrade to the newest OpenACS that is compatible with that version of PostgreSQL. Repeat.
If you have made changes to the core openacs packages, this can be a little trickier. If you have isolated your changes into your own custom packages, a little easier. You might need to create an upgrade script if you used PostgreSQL features that have changed, I find that is typically rare for custom code, an of course, the OpenACS packages should have the appropriate upgrade scripts to help along the way.
As you said testing your custom code alongside the newest OpenACS/PostgreSQL combo would be a good test to determine if you need to upgrade your code.
I hope this is somewhat helpful and encouraging.
