Thanks for the advice everybody. Consensus seems to be to bite the bullet and change out the coil. I didn't think of reselling it on e-Bay - thanks craig725 for suggesting that. It would make the bullet a bit easier to bite.
One more question: if I use the existing coils and the installer doesn't flush it out very well and I get some R22 in my R410 system, what happens? What can I expect to go wrong and when? I ask because he's offering 10 yr warrantee on compressor and 5yr parts&labor for the rest of the system (except existing coil).