I've heard a lot about an amp being British, American, or Blues voiced. I know that means the amp will sound best with British Rock, American Classic Rock, and Blues/Blues Rock. But can you change the voicing? If so, how? And does it make that big of a difference if the voicing is off for what you are playing? For instance, playing Blues through a British voiced amp

Please help an amp newbie
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Changing the voicing can be done, how hard it is depends on the amp. Usually it doesn't make sense to do so, because it doesn't matter if the amp is "off" as you say- there's nothing wrong with playing blues on a british amp. Lots of people do.
can you voice a vox ac30 6/tb to sound like a vintage 1963 vox ac30 like the one the edge from u2 uses.
The voicing is an integral part of the amp, but you can tilt it slightly either way by changing the power valves.

EL34/EL84 = British
6L6/6V6 = American

You have to get your amp rebiased though.