They can be removed and you can get new bulbs in there but it is very fiddly and takes a while to get it working unless your lucky first time. You take the old bulb out with pliers or by breaking them and bend back the contacts on a new bulb so that they push down into the holder. There isn't really a way to secure the contacts though to make sure they are snug and make contact correctly inside the holder, then you have to secure the bulb to the holder with glue or something similar. It can be done but it fiddly, I've done it with standard bulbs so I can't see why it can't be done with LED bulbs.