You have a couple of options regarding upper entrances. Some will drill a hole in the top super, just below the hand hold, others (myself included) will simply cut a notch out of one side of the rim of your inner cover. If you are concerned about robbing, simply place a small piece of screen over the opening.
For what it's worth, a beekeeper friend of mine acquired a hive and equipment, and the set up had a 3/4" hole drilled into the upper box (not sure how many in the stack). He reports that the entrance is well-used during the season, and that that hive did the best of his 6 hives, including over-wintering.
Thanks For all the insight. I'm starting another hive this spring and I think I'll go with a top entrance on this one. If I change the entrance on my established hive to the top, will the bees have issues until they figure it out?
Just reduce the bottom one a lot and have both entrances for a few days, then eliminate one altogether. They'll get it.
I keep both entrances going, but with a mouse guard reducer on the bottom one during winter.