Chuck's talking about the QSO not being in the log...I'm asking about the call sign used. Obviously the QSO is in the log just with the old call. So you invalidate the contact because the person happened to change calls since you worked em?
It really is pretty simple. If you send me a QSL card with a previous callsign on it and no notation anywhere on the card of the callsign you used during the QSO, chances are I won't recognize who the card is from. How am I supposed to know? Guess? When I receive a QSL card, I open my logging program, sort the log by callsign, and do a search for the callsign on the card. If the callsign doesn't show up I might
then sort the log by date and look to see what comes up at the date and time marked on the card. If the name on the card matches the name given during the QSO, I might
guess that you worked me with a new callsign and were too cheap to get new cards printed.
It is hard enough to get busy people to respond to QSL card requests. If you want to maximize your QSL return rate, then do everything possible to make it easy for people to respond. Sending out cards with the wrong callsign on them is a sure way to get ignored.