The game is featured in a Japanese slot machine called Pachi-slot.
The name of the game seems to be called "Dorisuta", and it features some characters from the anime series "Rio: Rainbow Gate". A pink-haired girl among them, named Mint, seems like she can transform into an older magical girl in order to fight.
Transformation scenes slightly shown in the PV of the game (1:43-1:50)
Actual transformation sequence in gameplay
Since she also appears in the anime "Rio: Rainbow Gate", I tried watching the episodes to see if her transformation was featured in it as well. Unfortunately, it wasn't.