I have a 13 year old 3.5 ton R22 system which now has a coil leak. The contractor is suggesting that I replace both the coil and the compressor. The new coil would be a R410a coil, but he says that filling it with R22 now means I can never it for R410a later. Clearly I prefer to not replace a working compressor. Does using a new coil with my R22 compressor really mean I can never use it with a newer compressor down the road?