Removing R-12-to-134a-adapter fittings is primarily dependent on the original conversion. Since these kits are sold in any autoshop they may/may not have been "properly".

Some may have been spot-brazed in place, others are simply screwed on. Some kits appear to have thread-locker on the new fittings while others don't. Some conversions only changed the low-pressure/blue/"fill-port" on the top-front of the engine to a 134a fitting and left the original R-12 high-pressure/red/"service-port" alone.

Modifications to fittings may affect the integrity of the schrader valve directly beneath it. The valves should have caps with O-rings in them.

Before attempting any conversion/de-conversion you really need to inspect the whole system and be prepared to invest in new components where necessary. This includes the proper operation of the heater and the cooling system of the car.

Some new components (e.g. R-4 compressors) may not honor the warranty if used in an R-12 "de-conversion". You need to decide if the warranty vs. performance trade-off is worth it when selecting what refrigerant you use.

The A/C lines and fittings, even though they're on an otherwise metric car, are SAE sized. Having the right tools is also very important.

You really cannot assume anything when you're working on the A/C.