I have heard different opinions on the rules regarding substitution of the DP or DEFO by each other. The way I read the rules (ASA/Rule 4, Sec.3,D<E<F) If the DP substitutes for the DEFO,(or vice versa) the DEFO(or DP) leaves the game bringing the total numer of players from 10 to 9. If the DP is re-entered this would be the only time it could happen.
The reason I bring this to the forum is I heard another umpire say that the DEFO and DP can flip/flop as many times as they want.