QBs, Kickers and Punters will still be required to wear 1-19, so if all of those numbers are retired or used by other positions, then someone will need to give theirs up to the new player. (How is a star linebacker going to react if he is forced to give his single digit number to a rookie kicker!)
Bonus: Which team could this happen on first? Well, that would be the New York Giants who have 7 retired numbers between 1-19 (1,4,7,10,11,14,16)