We did a conversion about a year ago from a custom AOLserver/Solid system to ACS 4.2 Classic on Oracle 8.1.6. We wrote a series of scripts to crank data out of Solid, possibly munge it a bit, and insert it into Oracle. We used (surprise, yet another shameless plug), nstcl. This let us use the database API with both databases. We could have configured an AOLserver instance to talk to both databases, but then we'd have had to use the old fashioned ns_db interface to one (or both) of them.
I'd be happy to share our scripts if someone is curious at looking at them, though since I imagine we're the only ones to have ever done an AOLserver/Solid to ACS/Oracle migration (and even if we weren't, no one else would have had our legacy data model :^) I don't know that they'd be of any real specific use to anyone.
I'd be happy to help with any effort to create conversion scripts for 4.2 Classic/Oracle to OpenACS/Postgres.
