Rich, I don't know if changing the signs was decided after the casino's initial opening (12/26/46), or if it was a part of their plan all along, but the second version of the sign coincided with their "Grand opening" March 1947.

The pylon is the same, so it's not a completely different sign - it just has different decoration, with the bird design added to the top. We know the sign was changed shortly in advance of the March grand opening because of two dated photos:

2/5/47 - Life Magazine photo, showing the first sign, and the hotel still under construction.
http://vintagelasvegas.com/post/40694405046

3/1/47 - Las Vegas News Bureau, showing the new sign.