If it works fine on phpBB then you have no reason to upgrade and waste $185+, vBulletin does have it's differences to phpBB and if you see you need those difference for your community's benefit then swap, otherwise as I said it's a pointless upgrade, not only does it cost money but it costs time, money can be re-obtained, however time can not
Why not? 1) Why not?, Giving your users better interface. 2) Why not?, Better Layout? 3) Why not?, Flexibility 4) Why not?, Security? Your community is properly established, Its 'great' time to upgrade it up to vBulletin. Good Luck!
He has a good point about security. About the passwords not tranferring over: LINK and phpbb version 2.0.4 - 2.0.21 » LINK. For version 3.0.0, passwords are not supported. It depends on which version of phpbb you are using.
I do believe I recently saw a mod/addon for vbulletin over on vbulletin.org (I'm not sure if guests or non vbulletin owners can access it) which claims to import users passwords from phpbb! If you don't want to use that then yes when someone tries to log in they are prompted to set a new password. I went from phpbb3 to vbulletin over a year ago and all my users had no problems with the transition.
no, they'll just have to request a new pass alert the members before any merge, make sure their email is correct and they won't have a problem.