Don't forget, you don't need to have a corresponding DPP to shift a player, as long as you have enough DPP's!!!!
I don't have a D/F, but wanted Sheezel in my Def line, so I did this:
My team, shifting Sheezel to the Def line.
Because you can't make a 3 way DPP change:
Davey traded out of my Mid line.
Constable shifted up to my Mid line
Sheezel shifted to my Def line
Knackers traded into my Fwd line.
Knackers traded to Davey in Fwd line
All 3 trades still available, and Sheezel now in my Def line