[BigJohnny]

styles for the most part will work across different versions, except in cases where the style itself has a lot of custom stuff that is dependent on a certain version.

3.5 to 3.6 for example, there was some stuff added in the postbit if i recall correctly, using a 3.5 style, wouldnt fully work on 3.6 because it didnt have the added stuff.

you can compare updated templates for your styles when you upgrade vbulletin, and if need be manually add the changed code to the existing styles. this is what i did for my 3.7 - 3.8 upgrade.

Modifications for the most part will work across versions as well, at least until the author updates them or you fix whatever might be wrong, which could be something minor, or something huge depending on the mod.