Newbie compressor question

I'm beginning the process of restoring my "Pepsi" Heintz Gullwing cooler.  Before pulling the compressor deck out, is there a way to prevent freon from escaping the system whene I disconnect the coolant lines from the upper unit?  Here are the compressor specs:
Refridgerator Unit Part No. 64346
Model A1 4FH
1/2# freon charge

Motor
Westinghouse 1/4 hp
1725 RPM  60 cyc.
110V   3.8 amp
40 deg. C, hours (Continuous)
Serial# FK    Style 956628A  Open

Thanks,
Bryan

If you unhook the lines to the evaporator you are going to lose the freon and there's little you can do to stop it. I haven't been in this spot before but if you must dump the charge you may want to find out what it's going to take to fill it back up. Can someone actually put in r-12? or must it be a substitute? If it's a substitute will it be as good?
I'm not familiar with your machine but many are configured such that the entire system can be removed without losing anything. If this isn't the case, and replacement of refrigerant isn't as good as what you have, you may want to consider creatively working around it.

Nate