I don't know if this is the right "string" to pass this on, but I saw an announcement a while back which stated that it was believed that the glitch preventing a team's DH from switching to a position when needed to cover an injury per se was fixed...just here to alert those who may be able to fix it, that unfortunately, it has not been remedied. I have seen numerous instances in the 80's and 90's games where my DH remains at DH at all costs...even if another player has to cover at a position he doesn't qualify, the game still does not move the DH to the field.
The glitch that got fixed applies only to the pre-game lineup, not to injury replacements during a game. (In the past, if you had Jose Canseco (for example) as your starting DH, and you set him as your backup LF, he would still start at DH even when your LF was unavailable for a game. That was frustrating.)
My experience has been that the DH does NOT go in the field with the mystery teams at this point, but it does seem to work for 20XX and ATG. You just have to make sure you only make 1 move (so your DH goes into the field for the injured player, and the DH is replaced). If you try to move 2 guys (so your CF gets hurt, you move your LF to center and your DH to left) then that wont work.