The reason for the failure is simple - apparently, head operating pressures for R134a are somewhat higher than those for R12, and that will lead to premature compressor seal failure.
If you've already done the conversion from R12 to R134a (including switching out the receiver/drier and associated componentry,) you should just need to change the compressor. It will be necessary to draw the system to a vacuum before charging, but if you have a big enough air compressor, you can get a vacuum pump for about $40-50 and do that part yourself. If you have a manifold gage set (they are different for R12 and R134a) already, you're probably going to come out ahead if you get the vacuum pump. If you don't, it will probably be cheaper to get the system done at a shop rather than buy the gages/pump/coolant/&c.
Don't forget lubricating oil - it's usually a good idea to "manually" oil the compressor before installation. Pour about a teaspoonful of oil into the suction port, and turn the compressor by hand. As the suction port dries out, add more oil in small quantities until you start seeing oil come out the discharge port. This will ensure that the compressor is already lubricated, and that you won't end up running it dry and setting up for premature working seal failure (and doing this job again next month.)
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