Answer:
Step one removal
you have to recover all freon from system then cut out the condenser unit and the evaporator coils and remove freon lines in between.
Step two replacing
once the above is done you will install the new freon lines via Brazing (size depends on the new condenser unit) you will need to retro fit the new evaporator coils to the old furnace (unless you have replaced that too) and install the new condenser unit.
step three checking for leaks
place vacuum pump on a line and pull a vacuum for 24-48 hours and close your gauges and turn of the pump check the vacuum make not of where it is (it should not be at 0) wait 24 hours and then check it again it should be the same (IE if it was 28 merc. then it should be 28 merc. after 24 hrs)
if not use nitrogen to locate leak.
step four charging
ALWAYS CHARGE IN LIQUID STATE
charge via super heat or use scales to get close to proper charge and then finish via super heat
step five finalizing
reconnect electricity, insulate liquid line(s), turn on and enjoy