I have went from phpBB2 ---> vB3 a while back.
The only issues I ran into was the #1 admin ID, everyone else's ID's reamined the same and everything else worked fine. I think if you went from
vB --> phpBB that it might change everyone's ID's but from phpBB -->
vB they did not.
Meaning, phpBB's defalut main admin account (the one created when installing the board) is userid=2. When you install
vB, that main admin id is userid=1. Therefore, after the
vB install and phpBB import, I had my main
vB created ID account as userid=1 (created during install) and the imported main admin account from phpBB as userid=2.
There are a couple options at that point. The easiest I think would be to just give the imported ID=2 account full and complete superadmin rights via
vB and just use that account as I am pretty sure all the existing images will match up to that account.
What I did though was I deleted the imported userid=2 account in
vB then using phpMyAdmin, changed the image owner ID in the photopost database of all my images to userid=1. Someone told me the SQL commands to change every instance of "2" to "1" in the "userid" field in the "pp_photos" table and it worked great. I thought is was someone here how helped me out with that SQL command but I searched here and @ photopostdev but could not find it.
I will add that I had about 250 members with about 3000 images in photopost at the time of the switch and the change was smooth except for the main admin ID 1/2 issue which was easy enough to remedy.