Thanks Jesse, it appears to be... logic !
After having tested today, I gave up this "undo-redo" solution to switch from verse to chorus and vice-versa, because the undos and redos cannot be quantized.
Here's the beginning of another solution :
1- I record Loop X for the verse
2- I assigned 2 midi messages to a switch of my footcontroller ("mute on" for loop X and "record or overdub" for loop Y) in order to record the chorus on another loop... SL accepts several midi messages from the same switch at the same time, that's great.
3- I then have to stop recording loop Y with a dedicated switch ("record or overdub" loop Y)
4- 2 other midi messages on another switch allow me to mute loop Y (chorus) that is playing and unmute Loop X (verse) in order to come back to the verse with a single switch. ("mute on" for loop Y, "mute off" for loop X)
5- Another switch allows to activate the chorus ("mute off" for loop Y, "mute on" for loop X) so that I can alternate from verse to chorus all along the performance.
For sure, you'll tell me that I should have used the "record or overdub next solo" midi binding at step 2... the problem is that I always record a 1st rythmic loop (Loop Z) that is the master sync for the other loops and that is always playing (both on verse or chorus).
A solution for that (for me) would be to ask Jesse for a special midi binding called "mute current loop and record or overdub next"
Do you think it could be possible in a next release ??