Sounds like your mechanic is not up to par on A/C systems as he doesn't seem to know what the gauges are telling him if he doesn't know what the high side reading should be. It might be time to get a second opinion from someone that can read the gauges.
Yes, freon will be released when changing either but have the freon evacuated before opening the system to change them. The system should be evacuated (vacuumed out) after you change the parts, to make sure there is no moisture or junk in the system and before adding freon as they can cause problems later. Jeepair is very knowledgeable and their prices are great if you can wait for mail order. (No, I don't work for them)
See another guy that can read the gauges and if might save you a lot of bucks in the long run.
My 2 cents