American Football IFAF Approved Ruling 1-4-2

The Team A player who started the game wearing jersey number 77
enters the game wearing number 88. RULING: The player must report
to the Referee who, without stopping the game or play clock, uses his
microphone to announce the change and the relevant flank official
informs the opposing head coach. If A88 fails to report it is a foul for
unsportsmanlike conduct.