Is it allowed to replace regular light bulbs with LED light bulbs?
I’m not talking about replacing the entire light fixture, just the bulb.
Are the regulations different for headlights vs. daytime lights vs. brake lights vs. backup lights vs. turn signals…?
I keep finding lots of merchants selling LED light bulbs - but I am not sure whether or not this would count as an illegal modification or not.
The change needs to be applied for, examined and approved by the DMV.
Not changing the fixture will lead to all sorts of problems, particularly for other drivers. The shape of the reflector you have is tuned to the light source, once you change the position and brightness of the bulb emitter that reflector not longer works correctly. Some bulbs claim to work in existing reflectors however “work” is a subjective word!
Is that more a formality or will they not usually approve such changes?
Yeah, not planning to change the headlights - just thinking about changing brake lights, turn signals and backup lights.
I changed the rear indicators on my motorcycle with these, much brighter than the original it’s been inspected 3 times since the change and nothing has been said.
I have them slightly pointing down so not blinding anyone, I know quite a few people who use the type below and put the defusers back on without any problems.
This is the way. 
