You'll need to have it retrofitted to run it off r134a. All up it costs just over twice the amount of a standard regas and it's a pretty common procedure. Once that's done, it's just a matter of having the system regased with r134a each time it needs it.
I had all sorts of troubles with my air con mechanic you may have seen in previous posts, so I'd definitely recommend going with someone reputable, as the age of the air con system gives the mechanic plenty of scope for talking you into fixing expensive problems which may not even exist.