An interesting observation... well, for me.
If my circuit with 2 modes has a potentiometer in it, and I perform AD conversion on its position, I get a different result when it tops-out, depending on which 'mode' I'm in... I assume this is because of the voltage drop that's introduced by the diode.
If I'm in one mode, then the potentiometer reads 0 when turned fully left, and it reads 255 when turned fully right.
If I'm in the other mode, then the potentiometer reads 0 when turned fully left, but it reads 255 when it's about three quarters of the way around.
So, if you turn the potentiometer to the point where it reads 255 in the second mode, and then switch to the other mode, the potentiometer reads 222 - and you've not moved it at all, the dial is three quarters of the way around and reads either 255 or 222 depending on what mode you're in.
I think I'm going to have to investigate the ADPREF bits of ADCON1 more... at the moment I've just set them to 00 when I do AD conversion and it seems this is not good enough.
It looks like "10 = VREF+ is connected to external VREF+ pin" is my best bet, but I'm not sure what I'm going to need to do to achieve that. It looks like, maybe, I connect pin RA1 of the PIC to the unmolested +5V coming directly in from the power supply? Or would I be better trying to figure out what "11 = VREF+ is connected to internal Fixed Voltage Reference (FVR) module" really means... I've never heard of FVR Buffer1 before.