A device and method for retrofitting a light fixture from use with a lamp socket that employs a conventional incandescent or metal halide lamp, to use with a light emitting diode (LED)-based lamp assembly. The lamp fixture has a collar with a base and an annular outer wall extending out from the base. The LED lamp device includes a neck base having an annular outer wall having a shaped outside surface that is placed into direct surface contact with the inner surface of the annular outer wall of the collar, to establish an effective heat-transferring interface. The shaped outer surface of the neck base provides proper fitting of the LED lamp device into the lighting fixture, and provides a heat-transferring interface over substantially all of the outer surface of the neck base, to dissipate heat away from the LED module. Aluminum material provides high thermal conductivity, light weight, availability, and low cost.