I tried this last night
I was able to complete it using the following steps:
Create a few versions of the enemy with the ball and one enemy without the ball. The skill used, triggers a common event, which turns on a switch. The switch triggers an event in the troop menu that looks at that version of the bird and uses the 'transform' action on the bird to change them into the no ball version of the bird. You can then use variables in the event to set the HP (or states) to be the same with the new bird.
So it would look like this:
Bird w/ Ball 1
Bird w/ Ball 2
Bird w/ Ball 3
Bird w/o Ball
Bird w/ Ball 1 skill : Drop ball 1 (Run common event Bird 1)
Bird w/ Ball 2 skill : Drop ball 2 (Run common event Bird 2)
Bird w/ Ball 3 skill: Drop ball 3 (Run common event Bird 3)
Common event Bird 1 : Turn on switch Bird 1
Common event Bird 2 : Turn on switch Bird 2
Common event Bird 3 :Turn on switch Bird 3
Troop Event page trigger: Bird 1 switch ON : Transform Bird 1 into Bird w/o Ball Turn off switch
Troop Event page trigger: Bird 2 switch On: Transform Bird 2 into Bird w/o Ball turn off switch
Troop Event page trigger: Bird 3 switch ON: Transform Bird 3 into bird W/o Ball turn off switch
Set this up for each troop with the bird, and you can copy and paste the event pages depending on which bird you put into that battle.
Great question! I added a monster to my game to do this for fun! Thanks!