When replacing a compressor in a home AC unit is it standard to replace the freon or is it possible to reuse the existing freon in the new compressor?

When you are replacing a compressor. Chances are that the compressor's internal motor had a burnout when this happens the there is a chemical reaction that makes everything very acidic this in-turn contaminates the refrigerant as well. The old refrigerant would be reclaimed and sent to a a reclamation center where it would be incinerated under the guidelines by the Dept of EPA .