When I was on advanced hosting for the very first time, I think I switched from PHPBB to MyBB. But I haven't switched this time around because I like PHPBB the most.
I started off with Zetaboards on my first forum, then I moved over to MyBB. That change was rough... I recommend never you move from a Free forum host to a paid one.
I had a phpbb3 forum that I converted to SMF (the island) and that pretty much killed it.
I have a phpbb3 forum that converted to vBulletin (wober) and activity has been booming ever since.
I went from SMF to MyBB. Twice. I've also gone from MyBB to IPB, then from IPB to vBulletin. I've also converted a couple boards out of phpBB, to SMF, etc. Overall, Impex was the only one which never converted my passwords, and SMF's seemed the most resource-intensive.
