So, anyway, back to the subject at hand. . .changing an R-22 OTR system to R-134a.
In order to do so, you would, at the very least, need to change the compressor and the metering device, since 22 and 134/12 run different pressures. You probably wouldn't have to change any coils, but you surely would have to run a good long vacuumn on the system to make sure you evacuated any air and residual freon from the system.
Personally, I think it would be a mistake to change it over, as 22 is much more plentiful and less expensive than 134a. About the only reason I could see someone doing so,
would be because it's harder to get certified for R-22 refrigerants than for R12/134a freons, so you may have difficulty purchasing R-22 without a card. The test for the automotive style freons is a simple, open book test and is inexpensive, whereas the test for full certification usually involves a seminar and proctored test, but it still isn't a difficult test IMO.
If you are thinking about keeping the 22, my advice would be to contact a refrigeration contractor in your area, as they will have experience with practically all types of freon, and might be less frightened of dealing with an OTR system using 22.
As for attempting to convert it, well, that will be very expensive and really will not solve the problem of leaks, unless the leak is from the compressor, which would be changed. Again, I don't know a lot about the OTR systems, so this is just my opinion based on my limited a/c knowledge of freons and the differences between different ones.