R12 to R134/A conversion?

My 93 F250's AC used to be fine, but yesterday, with a 106 heat index, IN TRAFFIC, it decided to pack up. So, I got the retro-fit adapter kit, 2 cans of the new refrigerant, and a can of the oil. I've heard you're supposed to change out the O-Rings, but I really don't feel like delving into the AC system. I just need it to last through the rest of the summer, I normally just leave the windows open other times. So, if I understand correctly, I dump the old R12 (I don't give a rat's rosy rear-end about the ozone, I know it's illegal) fill up with the oil, then top off the rest of the freon? I'm pretty certain I've got everything I need, I just want to know if this will work for another month or 2. Thanks for the help guys!