I talk about the Program Change from a layer.
It will be interesting to have a parameter to send the Program Change before or after a certain time (i.e 500ms) the activation of the layer.
I lost context here…
If the Part change happens via Part trigger time, you might just adjust the trigger time (Song/Part Trigger Times).
In any case, the current Part will continue to sound until all notes and pedal have been released, plus the time set in Preferences Layer/Layer Sustain. Unless you send the Program Change etc to the same Instrument and MIDI Channel that is currently sounding, it should be gapless.
Sorry,
Let me explain the context.
I have a Gtr effect pedal that I control in midi via layers.
I have 2 parts with a different effect on each part.
Now imagine my guitare starts playing at the second part but not directly on the first time of this part but out of time (not sure of this translation…)
So the midi message is sent when the second part is activated but the guitare has already start to play an eighth note before.
Right, I got that.
But how do you select the next Part? If you do it manually, well, we can’t predict when you want to do it and send the Program Change ahead of your thinking 
We might delay it, but you might just as well apply the Part change later…or I may still be missing something?
I will take a picture of my configuration.
I found a workaround. It is OK for me like this.