I'm sorry. Where does it say you can't bounce back and forth?
It's right here…this is directly from the online NFL rulebook…you'll see "Immediately" which is defined as pre-huddle in other sections as well as "he must participate in such eligible or ineligible position as long as he is continuously in the game, but must report still prior to each play"…the NFL recognizes that an offense could fool the defense and put this rule in to stop the strategy, but it wasn't enforced in this situation because Vereen switched back to RB with no penalty (on plays 9 and 10 from the 5 yard line):
REPORTING CHANGE OF POSITION
Article 1An offensive player wearing the number of an ineligible pass receiver (50-79 and 90-99) is permitted
to line up in the position of an eligible pass receiver (1-49 and 80-89), and an offensive player wearing the
number of an eligible pass receiver is permitted to line up in the position of an ineligible pass receiver,
provided that he immediately reports the change in his eligibility status to the Referee, who will inform the
defensive team.
He must participate in such eligible or ineligible position as long as he is continuously in the game, but prior to
each play he must again report his status to the Referee, who will inform the defensive team. The game
clock shall not be stopped, and the ball shall not be put in play until the Referee takes his normal position.