Yes, as Donna says, there is no problem whatsoever with reboarding, changing clothes, grabbing a snack, and then disembarking again. BUT talk it over with the Shore Excursion desk to make sure you have sufficient time between excursions. They usually start boarding an excursion maybe 15 minutes before departure time.
Honestly, if you have less than an hour between excursions I would be a bit antsy about trying to reboard between them. You have to get off one bus, go through security (which is not like airports, but still takes a few minutes), wait for the elevator, walk the length of the ship to your cabin, change, walk the length of the ship back, check out at the disembarkation, and then find your bus for the second excursion. None of this is any significant problem, but the little bits of time can add up.
Frankly I hate to rush about while I am "playing rich." I much prefer to have ample time to move in a leisurely fashion.