Well, speaking broadly, any forum I swap to will have to be able to import the following:
1. Users (and as many settings as possible)
2. Posts in proper order/format
I would prefer to grab as much as possible, but those are my "musts". Now saying that, most forums out there, including UBBThreads - have the ability to import and export all the major board brands Posts and Members. Some go beyond that and snag additional information.
So sure, you can go to most major forums and take your posts and members with you, just like you can come to Threads from another brand, and import your stuffage
