A friend has one and the upper is not compatible with AR lowers. You'd spend 3x the money to try to get it to work and still probably not have something shootable. Best bet is to buy the AR and get a conversion kit for it. Not sure who you heard it from, but if he insists on telling you that it is possible, just ask him.... Now where does the buffer go??
The 15/22 is NOT a ar15. The parts are NOT interchangeable. The uppers and lowers of the 15/22 have different dimensions then a ar15 uppers and lowers. The 15/22 is made as a 22lr rifle and you would likely blow the receiver all the hell if you did manage to machine a upper that would fit it.