rogerd - 12:07 am on May 30, 2011 (gmt 0)
Migrating a community can be a big job, and even if the new platform is infinitely better than the old one, people will complain.
Start by listing the problems that exist with the current site, as identified by you as well as members. You'll want to be sure the new platform can improve most of these, and you can also use the list in explaining the change to the members.
Migrating user IDs and logins is very important, IMO. You'll have chaos if there's a landrush for new names. Migrating threads, posts, and other content can be tricky. Some platform combinations have migration tools that make this easier.
You can also leave the old content in place (if it is getting lots of search traffic) with plenty of linkage to the new community.
My advice would be to only do this once - choose your new platform to be highly scalable and likely to meet your needs far, far into the future. The bigger your community gets, the more traumatic future migrations will be.
I migrated a busy community from Discusware, on which I had hit the wall capacity-wise, to vBulletin years ago. It was highly disruptive for a period of weeks, but eventually was well-accepted.