Depending on how many template edits the theme maker has done it could be an easy upgrade.
For the most part upgrading to 1.2 is super easy off the bat, fine tuning custom themes with the responsive part is the time consuming part.
Main things to check...
Style Properties -> General
HTML/BODY, remove any min-width/width settings
PAGE-Width Controller, set max-width instead of a fixed width
PAGE_CONTAINER needs to be updated(it calls the responsiveness)
Default xenforo CSS templates usually have custom responsive CSS at the end.