Its a definite no no. Completely different valves and fittings needed in your system for a start. Dont even think about it they dont mix.

I very much doubt R12 is illegal but the point is that no more is being manufactured worldwide since its disposal is damaging to the environment. The existing stock is still circulating and able to be used. R12 seems to be much more effective than R134 so if you need a small top up then try and find some more. If you do decide to convert then make sure someone pays you for the R12 they take from your system as it is a valuable commodity.

Keith