I've heard all the rumors comparing r34a to r12, and how r134a does not compare to the classic mix. For myself, my 92 ford pickup lost the compressor about three years ago (r12 system). So I considered my options, and converted it over to r134a - right in the driveway. As previously mentioned, you simply flush out the lines - the two refrigerants are NOT compatible - and since I replaced the compressor, I also replaced the drier (er....or whatever the name for that cylindrical unit is....). And the best part is that I can now service the system myself, at least for now - as the refrigerant is available to you and I over the counter.
I'm happy with how cool my truck’s a/c system is blowing (I'm in Dallas - and you can NEVER get too cool here). So, there you go. Mebbie in a vintage system the r134a is not as effective.... However, I'm sure r134a will soon go the way r12 did, but at least, if you converted, you could stock up on the more affordable and available refrigerant.
70 Mach 1 (351C 4V 4-speed) I've been restoring since '96. 95% complete. Also have a '68 HT (289 2-speed) that I restored between '97-98 and is FOR SALE! The VMF has proven to be an invaluable resource for information, humor and excellent advice.