this is my guess - i'm probably totally wrong. values and changes are hypothetical.
let's say you have Bass set to 5 in FW14.
you upgrade to 15.
your Bass knob still says "5" - but because of the new firmware, 5 is actually now 3.
so you think your bass is on 5, when it's actually 3. you could go through each parameter one by one and change it, or you could switch amps away and back to do the parameter reset.
you do the switch, and though the Bass knob still says 5, it sounds very bassy now, since the new equivalent would be 3.
so you move the Bass knob until your ear tells you it's right, and now it's at 3.
regardless, i just do the amp switch thing and move the basic controls and i've got my tone in like a few minutes.