Not a good idea using R407c as a drop in for R22,without an oil change and a system flush. R22 is designed for mineral oil, R407c is designed for use with synthetic oils. The chemical reaction between "new" refrigerant and "old" oil, could lead to waxing of the oil. You could be lining yourself up for a massive headache! That's not to mention the potential miscibility issues...
As a general rule (although not really relevant now as all the "old" refrigerants are banned) You can use the "old gas" on the "new" oil ("ester" / akylbenzene), but NOT the "new" gas on the "old (mineral) oil". (Well, that's how I remembered it back in 1995!!)