Thanks Jason,
I figured there was a slim chance that the loads of the two lamps where close enough to be compatible; at least in the case where advances in led technology result in brighter output for the same wattage.
And in this case, the Plus model specs still rate the lamp at 20W. So, electrically I'm not sure what's not compatible. But physically, perhaps it's a different form factor.
Anyway, given the apparent improvements with the board, firmware, etc., I may as well unload the units I have.