During the position change stage of the off season, move your selected reciever to tightend.
Temporarily leave the stage by pressing B and accept the changes you made.
Re-enter the stage and move him back to reciever.
His speed and acceleration may be over 100, but it will be trimmed to 99 in the training results stage.
If you perform the cheat on multiple recievers some of them may not be trimmed until the next off season. Their stats may say 100+, but their game speed won't show it.