Early LED Maglites were just regular incandescent lights with an LED drop in replacing the incandescent bulb. They included an incandescent bulb in the tailcap foam.
Later LED Mags had a permanently installed LED (part of the tower rather than something that dropped into the tower's bulb socket) and eliminated the tailcap spare bulb.
If you are able to remove the LED assembly (by unscrewing the collar ring as you would to remove a bulb) then you should be able to drop in any LED conversion bulb designed to run on the same voltage.
If, on the other hand the LED is built into the tower then that gets more difficult. Can you give us a bit more info ?