Well, SMF does provider converters to port over the data files in PHPbb to SMF if that is what you need. See http://www.simplemachines.org/community/index.php?board=133.0 However the designs cannot be ported over.
Yes it is possible through convertors, you will have to strugle with it and you will loose some links in search engines unless you know how to redirect them. but it is all possible
Vbulletin has nicer hacks and looks better, but SMF is free and I think more stable. SMF is also more secure and more spam proof.
MyBB is easy to use and very powerful. The only reason it is lesser-known is because Fantastico doesn't install it, otherwise it would be the #1 free forum. Try MyBB once, check out the features, and you might never use the lame free forums ever again. Note: You can convert both phpBB and SMF to MyBB.