How many quarterbacks have lost and then won a Super Bowl?

Len Dawson of the Kansas City Chiefs lost SB I and won SB IV.

Earl Morrall and John Unitas of the Baltimore Colts lost SB III and won SB V.

Bob Griese of the Miami Dolphins lost SB VI and won SBs VII and VIII.

John Elway of the Denver Broncos lost SBs XXI, XXII, and XXIV and won SBs XXXII and XXXIII.