The old forum should be turned off for new posts the day this forum opened.
Several people have already discussed how vBulletin has a tool to transfer the Mysql database from other forums like phpBB into vBulletin but it may not be a perfect transfer do to forum sections.
There are also skins, themes or styles depending on which software you are talking about that are available mostly free. Take for instance this forum looks like a standard forum with some color changes to tables but no one has updated the pictures,icons and buttons to match so it looks like a beginer website. Anyway this has all been discussed on the old forum... HTH