Would this actually be a problem though? The goals and rewards for the positions that you'd actually switch are pretty similar. Like are they afraid people are going to put Calvin Johnson at LB and run nanos with him to get sack goals?
Who cares? I guess they could make XP use it or lose it before switching positions, but you can still upgrade most of the same stats anyway, right?
Anyway, in my opinion it's not worth leaving out a basic thing that happens regularly in the NFL like switching a player's position in order to stop a few people from doing ridiculous things to gain a little extra EXP.