Right. I'm already planning to take it to a shop to evacuate the system and then do the recharge, after I put the new parts in.The R134a currently in the system needs to be removed and saved before you open up the system. Even though R134a does does not effect the ozone layer, it's still a greenhouse gas and a cause of global warming.
You need to pull a vacuum and make sure the system can hold it for an hour.
Then fill the system by weight.