if the existing line set is properly sized and re-used for the conversion to r-410a equipment, after flushing the line set is there a test that can be performed to ensure all residual oils have been removed?
IMO, if you want to change refrigerants, change the line set as well.
also IMO, i would venture to guess there will be an approved drop-in replacement for r-22 in the future. whichever company produces it will hit a gold mine.
one other question. how long will 410 be the industry standard before something else is developed?