I imagine that a mix of r12 and r134 will mess up the phase changes (i.e., liquid to vapor) needed for a/c to work efficiently. Then there's the issue of oil compatibility. Some oils are compatible with both refrigerants. Most aren't. Compatibility means that the refrigerant will carry oil and circulate it throughout the system. Otherwise the oil just sits at the low spots and doesn't lubricate.

Converting to HC refrigerants is legal in many states. But it can only be done on r134 systems. It cannot be done directly from r12. An r12 system must first be converted to r134 (with proper quick disconnect fittings), and only then may it be converted to HC (at least that's your story when the a/c police ask you about it).