How to use/test HEAD version?

I have upgraded a site from D5.4 to D6.4, but how to get private messages back?

I tried installing HEAD version (update.php updates nothing) -- the module doesn't work (messages are said to be sent but are not received, nor the old messages are listed). On the other hand, there are too many patches in the issue "Port module to 6.x" to understand which one to use, and whether any database updates are implied.

Is there an upgrade path (from D5 database layout) to try to use HEAD version?

Thank you.