OK so after some experimenting and… math! I understand that in polyrhythms signature mode changing the upper number of the time signature does change the track speed, as if there was a BPM multiplier in effect: eg. changing 4/4 to 2/4 halves the BPM multiplier (and the track speed), changing 4/4 to 8/4 doubles the BPM multiplier (and the track speed), however it also modifies your notes!!!, eg. when changing 4/4 to 2/4 the existing note durations get halved, when changing 4/4 to 8/4 the note durations double. So the resulting effect is that the notes play with the same speed.
So to be able to actually change the note durations it would be sufficient to remove or disable either the existing notes duration modification or the BPM multiplier (track speed) modification when the upper number of the time signature is changed by the user
(The polymeters signature mode only changes the track length when changing the signature and doesn’t change the BPM multiplier, nor the track speed, nor the note durations.)