Hmmm, when you are low on refrigerant, the system *should* still cool, but not as much. If it suddenly went warm, I was suspect the problem is elsewhere. You should try and get some codes out of the A/C system.
1. Put the key into the ON position (position 2), but make sure the car is off.
2. Put the climate control into AUTO and LO.
3. Press and hold REST and DEFROST (front/windshield defrost) at the same time until you read dIA on the climate control screen.
4. Using the AUTO button, scroll through the codes, and record them. Report the codes back to me and I will tell you what they mean.