Toyota Repair: 92 Toyota 4 Runner A/C, mile warranty, toyota 4 runner

Question
Just bought a rebuilt motor and had it professionally installed.  It comes with a 3 year 36,000 mile warranty. My problem is that yesterday I turned on my A/C and its blowing warm air (It worked when I took it in there).  I have a feeling that the mechanic will try to deny responsibility. Does the A/C have to be discharged first before removing an engine?  What information can you provide me to be armed with before reporting the problem to them?

Answer
Since it worked before it's a pretty good indication that they forgot something, the a/c system doesn't have to be discharged when replacing the engine, the compressor can be removed from the block with the lines connected. They may not have reconnected an electrical connector,just have them recheck it for that, it's probably something simple.