When you shift into other forms, it's a clear transformation. A temporary move like what Pac-Man does isn't the same as full-out transformations. Digimon's whole point is taking another full form, so.
Switching between partners is identical to what transformations are.
To be clear, Transformation character means swapping in gamplay to another full character. If the other character is always active, you likely can't control them. If you can control them, though, it's a bit different. That's often called a Puppet Character. Basically there's three types for multiple characters in some way;
- Transformation: Directly swaps to another moveset.
- There's no clear name for having another character onscreen with you that fights with A.I. You don't have that much control in all cases. However, it's clear you still partially control their actions at some point.
- Puppet Character: You fully control two characters at once. Their A.I., if any, is generally to counterattack once in a while and then come back to you(with or without respawning). These are super simple A.I.
I'm not sure how Garurumon and Gabumon would fight here. Are they going to mirror your movements, but often attack on their own? Or only do stuff when you say so by default? Obviously it's still a transformation character in context, but it entirely depends upon how the A.I. works.