It would certainly be a good idea to move all existing (user) content and users to the new framework and keep it on the existing domain. Or at least the postings/threads and users. As for the passwords, there could be an easy solution: Implement in the old Everything engine (which we'll keep around for some time in readonly mode) a function to "activate my account on new perlmonks" as some kind of password change function; except that the user athentificates against the EE, and changes his/her password on both sites (which also locks the account on EE and unlocks it on PM::New).

As for Perl 5 vs. 6; i'd probably implement it in 5 with existing and tested frameworks (waiting for a stable framework in 6 for years without getting PM into the 21st century doesn't sound that good an option). Since quite a lot sites and projects depend on 5->6 interoperability and smooth upgrade options anyway, we'll be able to port the site step-by-step over time to 6 anyway (i hope).

Of course, this would all be depending on the good will of the gods. But as long as they don't have to do all the work (and keep their goodly status), i doubt they will be very much against the project ;-)