Hi Mike; if you change from a 250k tone pot, to a 500k tone pot, the new pot will work exactly like the old one from 0 to 5. So basically, you've added the extension of 5 to 10. You can "dial-in" a bit more treble, or technically speaking, shunt less of it to ground.
I would disagree that the "quality" of the control will affect the sound. 500k is 500k. The quality will only affect how the pot "feels" and how long it lasts.