Can the old and new type of gas be mixed together and if not what will happen in the operation of the heat pump

Are you talking about Freon? If so, No they cannot be mixed as they are different bases.

If mixed what can happen with the operation of the unit---would it fail to operate properly in providing heat through the coil or gum it up clog it etc.

Hey Grammabee, they are different ester bases and will not mix. In addition, the newer Freons operate at different pressures that your old compressor may not be able to handle. If you have an old unit, R-22 is what was probably installed and newer units use 410 I think. You may be able to find some 22 around but it cannot be manufactured anymore. Also consider that the newer heat pumps are much more energy friendly so you may save some money there.