is there a specific reason players listed at DE can be inserted into the DC to play LB, but LBs can't be played at DE? i know it's been that way forever, but figured i'd ask why, and if that's something that might be changed?

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It is the same way with WR and RB. RBs can be slotted at WR, but WRs can't be put in at RB. Also with FS and CB.

You can unlock the depth charts so they can play anywhere. Locking the positions was more of a realism thing to keep people from just playing all LB at every position... it would force you to change their position first... but now that we have skills that drive a lot of that it may not be necessary to keep it locked.