I think a good suggestion for further development could be to divide the matrix in position eligibilty and player conversion. That would result to convert a CB into a Safety but not to position him there as long as he hasn't gained a certain Minimum weight for the position.
This could be a solution for any position

For the overall game, the minimum weight thing might a good thing to add, so pass that along to the beta folks.

Recall that SF did propose the extreme position limitations during our rules discussion, but he was out voted. We kept the "play any OL" and "play any LB" but DL and DB we did try the position limit. So I guess if you don't like the current rule, blame SF, the biggest cheerleader for it!