Some additional info:
1) If you require support and have modifications installed, then you might be prompted to restore the default vBulletin in order to receive support as you will not receive official support on modifications. This sounds more difficult then it is. Most modifications these days use the Plugin system, meaning that no file edits are required and they can be turned off by flipping a switch.
2) As already mentioned it should not be a huge issue unless base datastructures have been changed. You will need to keep in mind that custom data from your current phpBB modifications will most likely not transfer to vBulletin with the standard import script.
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