Actually, you can use the aux switches for +/- preset change in play mode.  You just need to assign them to FS1, FS2, and FS3.  These assign the opposite mode functions to the aux switches.  So, when you are in Play mode, the aux switches have Bank mode functionality and vice versa.   You can also explicitly assign Pre+ and Pre-  to aux switches as well. 

Apologies if this wasn't clear from the manual.