A shadow screen transformation with some careful choreography can be very dramatic.  Alternatively, a projection of the transformation while the beast himself is recostumed can be good.  If you want it all to be conducted a-vista, you want the beast makeup to be confined to mask and hands.

So here's one take... Beast falls to ground behind a scrim.  Beauty reacts in grief centerstage in the audience's line of vision. Projection of the flower with the last petal falling on the scrim, while people gather round the beast.  (People are actually recostuming beast, while hiding him).  Sad music then transitions to miracle occuring music.  While that music is playing the projection on the scrim changes to be psychodelic swirls or falling stars or something that indicates momentous change. Choreography can even involve lifting the body of the beast overhead, allowing the costume to be removed. Body language of people changes from mourning to surprise and shock.  They whisper and talk as they draw back revealing the Beast-now-Prince who awakes to himself. 

Consider using a fog curtain to mask an actor change - one in the beast costume, and one in the price costume.   In our case this worked well because the staging included a tower that rotated from the bedroom where the rose was to a balcony.   After being stabbed on the balcony, the beast falls through the door to the bedroom and Bell enters.    While he is falling through and switching with the other actor, the tower rotates to the bedroom scene and a fog pipe around the base of the bedroom begins pumping out a large fog curtain.   He then rises out of the fog as a prince.