Am I correct in assuming that the blockhain DB is using LevelDB and does not depend on libdb? And is therefore wallet.dat the only critical file still using libdb? If so, I can easily migrate simply by exporting+importing the relevant addresses. Or is there something I'm missing?
Yes, wallet.dat is all that uses BDB anymore. If you dont mind losing your address book (though you should be able to export that with bitcointools or something) you can just export your privkeys and reimport them with a BDB4.8-based build.