Yeah, well first you have to make a variable which you set to current SP, and then when that is under 100 you switch back, because there is not an option under con. branch which does that directly.
Edit: I guess if you are going to use some new parameter which controls transformation, then it would be con. branch with a script condition based off that script. You could make a stat like that from in game variables, but I am not sure how to make that show up in the battle screen, because I have so far been unable to determine how to display the value of an in-game variable through a script. So... I guess wait for the script.
The second might require a tiny script. I'll have a a look when I get home.