Flint - do what I intend to do.

Back everything up TWICE - and install a 'dev' copy of the new software so you can tweak it to your needs.

I have various other programs implemented alongside my Threads database.

[*] XML pulling in from various websites and sync'ing with U_Number
[*] Photopost (which will be borked by this new system) which links by U_Number
[*] A Member management system I developed that also syncs via U_Number
[*] etc. etc...

So this new system will TOTALLY screw me. But knowing that, and knowing it is just a matter of rewriting the way everything maps, I will keep 6.5 running as my main, while using my 'dev' install to re-hack and re-sync everything using duplicate copies of my DB and files.