Welcome to the Forum John:
First you will need to know if the current evaporator coil will work with the new systme or if it will hamper it.
If Trane says the same evaporator coil will work, you only need to pull a vacum on the old system getting all the R-22 out; then the new refrigerant will work just fine. There is very little of the oil contained in the system; most of it being in the compressor (where it belongs). However, any oil in the system will also give up the R-22 when a deep vacum is pulled on the whole system, which is required in order to make the change.
Glenn
