If he installed a rebuilt compressor then he should have flushed the system before installing the compressor. Most all auto parts won't warranty a compressor unless you can show them proof that you flushed the system and replaced the dryer and orfice tube. Sounds like the mechanic messed up and is trying to cover his tracks. If it's building too much pressure then you need to try this first, take your car to the car wash and pressure wash the condenser in both directions. A lot of times it's just got a lot of bugs or grass build up in the fins and that'll keep it from cooling and cause it to build pressure. If this doesn't work let me know and we'll get together and unhook the conderser fittings and flush the condenser, then hook it back up. A/C flush is about $25.00 a quart and 1 quart should be more than enough. The orfice tube is less than $5.00 and can be changed while the condenser is being flushed, and if your going this far go ahead and replace the dryer for about $30.00. This should get your A/C working like new. By all rights it shouldn't cost no more than $60.00 in parts then your oil and freon, and the labor shouldn't be more than $100.00. Total about $200.00 to stay cool.
As far as switching from R12 to R134a, all you need to do is evacuate the system and change the a/c oil add your new fittings and then charge the system with R134a. You don't have to change anything else. But why convert it?
President
East Tennessee Fiero Club
Robert Finley
RobsFieros@comcast.net