The only trick to this is that when you put the screen number to change to in the PLC to Screen tag, then watch the Screen to PLC tag for a match with that screen number. After it matches, then set the PLC to Screen tag to a number that is not a used screen or zero. That way when you put a new valid screen number in next time you want the screen to change C-more can see that the value has changed.
It is really that simple.