There is a special tool which can remove the schrader valve without losing the charge. However, the r134 valve adpapters that I have used in the past instruct you to remove the schrader valve before installing the adapter. To do this, of course, you will lose the charge. You won't be able to work quickly enough or safely enough to do this without losing the charge.
The good news is that you are losing 134, not the gaseous gold R12. If the system was correctly evacuated during the changeover, you should be able to blow down most of the charge, put on the fittings, then evacuate and charge. It's always best to pull a vacuum as long as possible, 24 hours if you can. This removes the maximum amount of water molecules. If left in the system, the water molecules combine with the refrigerant developing an acid that will potentially eat up components from the inside out.