Your local auto a/c shop will have a machine to recover your R12 and recycle it for recharging after you've done the flushing. Be honest with him though and let him know that it has Ester oil. He may want to use a separate cylinder from his usual recovery cylinder.
I have two recovery machines, one set up for 12 and one for 134. On my machines the oil is separated from the refrigerant during recovery and I don't think it would hurt anything.
Just be honest with him so he can protect his equipment and keep from contaminating our remaining R12 supply, part of which is what we can recover.