#1
Is it necessary to change the pots when switching from single coils to humbuckers?
#2
It's usually a good idea. You don't have to, but there's a reason it's standard practice.
The pot value change makes the pickups sound different, so if you find your humbuckers sounding dull and muddy and with less output than you expected, you'll get to go back and do it right the second time around.

It does depend a lot on the individual pickups. A JB is perfectly fine with a 250K pot, an Alnico II Pro is probably going to be messy with the same.
#3
Lower pot values roll off the top end a bit but gives a smoother taper when you turn the pot up or down. Single coil pickups usually use 250k pots because single are already bright and the slightly reduced top end is a welcome improvement. Humbuckers are darker which is why they tend to have 500k pots. Some high output humbuckers even want 1m pots but it's all about the top end.

I'd change the pickups but keep your 250k pots and see how it sounds. If it's too dark or seems muddy then get 500k or 1m pots. Remember there is no set right answer. Just because it's what most people do doesn't mean it's what you should do.
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#4
Quote by reeros321
Is it necessary to change the pots when switching from single coils to humbuckers?


I change them. Most single coils have 250k pots, most humbuckers are running 300-500K pots (500K is most common, though Gibson ran 300K for a while).
I find that 250K pots make humbuckers muddy-sounding.