If you change the outbound leg of any ticket, ever, the entire itinerary has to be repriced to todays price. Thats a rule anywhere.

In order to price the return you need to see if DL revenue management (RM) will allow a temporary oversell to allow the inventory to price. If they will then they might be able to duplicate the space long enough to allow for a reissue. Youre going to need to find an agent that not only cares but understands what youre asking.

My suggestion if they wont do the above, is:
1. Ask Delta to allow you to downgrade the return leg to whY, then reissue. This will be cheaper and will result in a voucher being issued.
2. At the same time ask Delta to grab the space you release when you downgrade the return leg(again in order to make sure this works an oversell allowance would be the best, but you dont NEED it) and then have them regrab it in a new record and apply the vouchers and pay the difference back up to F. Ticketing support will be required for this.

To be fair-this has nothing to do with Delta, its just a ticketing scenario.