Probably depends on the type of conversion fitting.

My low side conversion fitting actually left the old schrader valve in the old R-12 port - it was basically just a collar for the 134-style adapter. But the adapter was threaded for a schrader. You should be able to just unscrew the old schrader and replace, you may need a longer schrader adapter than the standard tire valve.

But - you'll need to evacuate the 134 first - or find the thread about the "don't evacuate swap your valve gizmo tool" that was on the forum a few weeks ago.