I would avoid attempting any change on an award ticket or in fact any type of rebooking online as delta.dumb is pretty messed up and people attempting to fix schedule changes have reported losing flight that they wanted as soon as they began to investigate alternatives on line. Call DL and be very clear that you want a specific change without canceling the rest. Do not be afraid to hangup and call back if you are not confident that you are talking to a competent agent who understands your request.

However, if you are changing from two award tickets to one there is a chance that the price per ticket in miles should drop, so try to ascertain that you do not pay more miles than necessary for the one award ticket that you want.