as said numerous times in this thread, the knob values of 14 don't necessarily correspond to the same positions in 15. so if you had Master Volume at 6 in 14, if you put it to 6 in 15, it may not sound the same because it is a different firmware. (that's just an example, the numbers and relationship are not correct.)
so even if you matched all the previous values of advanced and basic parameters, it will probably sound different.
the best thing to do is to reset the amp as you mentioned, then use your ears (not your eyes and the numeric values) to change parameters until it sounds like what you want.
if you are in the middle of a session or don't have time to change your tone and redo it, it is highly suggested that you do not update the firmware.