Hi!  We recently had to make the half day vs. full day decision for our son.  My best advice is to talk to your child's preschool teacher about which she thinks would be best.  My son is a "young five" and so I automatically assumed that half day would be best for him.  But his preschool teacher actually convinced me that full day would be better for my son.  (In our school district, full day kindergarten does not cover more material than half day.  It's the same curriculum but full day students get more time to do each activity, have more recess time, more free exploration time in the class room, etc).  Our preschool teacher said that our son was ready for the longer day in terms of his energy level and his social skill and that he would be a lot happier to have more time to work on each activity.  I think she was right because he's had a great year in full day kindergarten.

I can't really address the second part of your question because of never heard of that situation.  (Again, in our district full and half day cover the same material so the full day kids don't learn more than the half day kids).   I will say though that I think it would be hard to make a switch from half to full (or vice versa) in the middle of the year.   In our town, there isn't extra space in any of the classes so where you start is where you stay.

Good luck and let me know what you decide!