When I last attended SDCC back in 2010 I only went (Friday and Sunday)
I asked them if I could get my Sunday Badge as well when I was there Friday
They told me they don't do that, I would have to Pick up up the Day
The Badge is good for - because of Color and Day that was at Bottom of the badge.
of course maybe they changed this policy - would be great to pick up both badges on same day. although when attending SDCC 08-2010 i only spent like 10 minutes in line because the registration teams were so awesome at moving people through! 07 was 3 half hour wait though