HID's are rated 3,000 hours +
LED's are rated 10,000 hours plus.
By the time the LED's burn out, the lens would most likely be shot, all hazed and needs replaced.
There have been LED burnouts, a guy at work had a 3rd brake light, one led on the strip died.(VW Jetta)
I have also seen led burnouts on some Chrysler badged cars. It is rare but does happen, and yes you replace it as a unit. There is *no* standard when it comes to LED sockets that I am aware of?
In headlamps,
The ones I'm familiar with have a plastic plate, that houses the LED, and two small screws that hold the plate to an aluminum heat sink. I don't see why you just can't unscrew it, get an LED and some paste, and pop one back in. The only thing keeping you from doing that, is finding a LED, and being able to remove the front lens. I'm talking like vehicles like the Ford MKZ LED's. Projector LED's may never be serviceable... but Projectors should be obsolete if U.S. officials adopt new euro lights with radar sensing beam adj. tech. using multiple LED's to shape beam pattern automatically per on coming traffic.
But... the next Toyota Camry design will have a removable muti-colored lens using a gasket, perhaps to service the LED's, who knows.