The exported style is actually an exported PHP array that holds all of the classes and their associated settings. You'd have to hand edit the actually exported style and then reimport it, which is a bit tricky as it will break if the php syntax isn't correct.
When doing the upgrade, you can actually reopen your forums before worrying about updating your styles. Basically, it won't break anything, the new classes will just be unstyled until you edit them, so you don't have to extend the downtime.
That's what I did here when I updated