I recently read an article in a local HVACR magazine that somewhat addressed this problem, which I guess we all should have foreseen coming because R22 is getting very scarce and its price is skyrocketing.
According to the article, many contractors have started adding R410a to R22 systems that have experienced refrigerant losses. Apparently, contractors say they have been told by their suppliers and even some refrigerant manufacturers reps that this is a viable plan. The article also states refrigerant manufacturers are simply discouraging the practice.