For one of my teams I changed a bunch of RBs, a FB and TE all to WR simply for weight reasons.
Right now, the only method at our disposal to have any influence over a player's weight is their position, and unfortunately speed is directly related to weight.
The players I changed were already fully developed at their "normal" position, so I am not losing anything skill-wise by making this change, but I am able to get them to lose some weight, thus becoming faster.
I wouldn't do this with a young player who still had room to improve at their core position skills