Subs with DP/FLEX

[Dakota]

ASA or NFHS rules (take your pick).

OK... I've been going around and around in my head about this. Maybe I'm a victim of my own convoluted logic in trying to make up DP/FLEX scenarios to help people think through the options, legalities, etc. But here is the conundrum I'm up against...

Situation: DP is out of the game, and FLEX is playing both offense and defense. A sub is entered for DP/FLEX as the team is taking the field. FLEX player remains on the bench.

Question: Can the sub be designated as the DP playing defense for the FLEX, or does the starting DP have to reenter first?

I can't find anything in the book that would say the DP has to reenter first, but one of our esteemed web blues says she does.

Am I missing it?

And, if I am, what about if the sub enters while the team is on offense?