I am pretty sure you can, but since I never have, I can't swear to it. Hopefully someone will come along that is positive.
Even if you can't, you can still use e ticket to get into the park from that entrance and just do it at the Guest Services location by Spaceship Earth whe you make your way around, to the front of the park. When you upgrade your ticket, it is based upon the full value of your ticket when purchased; the do not prorate it based on the number of days already used.