The correct thing to do is to take it to an AC place and have them recover the gas which would otherwise escape as soon as you broke the seal. After fitting the new compressor you'd have to get it re-pressurised and lubricated to spec.

If it's still charged with CFC (R34 I think) it's illegal in Australia to release it to the environment as it's ozone depleting (in fact so are HCFC R134a but they ave a much shorter half life). Not sure on the legislation in your part of the world however I would strongly urge you to have it recovered correctly.

If it's the new gas, R134a then the harm in letting it go is not as bad but still not ideal.