Oz,
I suggest you go to www.delta.com and run thru a dummy booking of those same flights and bring up the booking terms & conditions and read it in black & white if one must travel on those dates/flights etc.
It won't specify your precise situation but if it says, to the effect, that flights cannot be changed/amended, or they can be changed/amended for a fee of X amount, then that means much the same as the passenger must travel on the outbound.
Do not take an individuals word for it, read it in black & white for yourself, print it out, because one individual will give you a different answer to another individual.