Ah-Ha! It finally dawned on me that spinnaker might be talking about the comparators when he mentioned polling the analog channels. I looked it up, and in fact, you can set up the comparators to interrupt the processor when they get a pre-determined value. And so, the OP can do exactly what he asked, but use the comparators instead of the A/D convertor. Though, using the A/D would most likely work also.
No I am not. See RB's post. This is exactly what I meant and pretty much what I suggested way back at the start of the thread except I did not suggest using the interrupt on change (as I said I am not sure it is really needed and adds complexity). Maybe my use of poll was misinterpreted. The purpose of the conversion complete is just that. It interrupts on conversion complete of the ADC. You still need to hit the analog input over and over to get the conversion to start.
All the conversion complete interrupt is to keep your MCU from being tied up watching the GET_DONE.
Again. if OP wants a interrupt on change then an external comparator will need to be used, attach to the external interrupt pin. That way the only time a read of the ADC might need to occur is when an external interrupt is triggered.
The 18F does not have a comparator as far as I know. It has a capture and compare but that is for measuring pulse width.