Here are the "Pay With Miles" refund/Reissue rules from website:

Refunds/Reissues:
Except for tickets refunded under Delta's Risk Free Cancellation policy, miles redeemed with Pay with Miles cannot be re-deposited in the member's SkyMiles Account - even for fully refundable fares. In the event of a ticket being reissued, any residual dollar value between the original price of the ticket purchased using full or partial Pay with Miles and the reissued ticket price, can only be returned as a non refundable e-credit valid for future transportation. E-credit will be valid for 1 year from original Pay with Miles ticket issue date. Residual e-credit value will follow current Delta e-Credit rules and will not be applicable on non-transportation fees. Pay with Miles unused tickets can be used as an e-Credit when it is a primary exchange document. All Pay with Miles terms and conditions will apply to any ticket purchased using an unused Pay with Miles ticket.

In the event of a refund (including fully refundable fares), only the paid portion of a refundable fare will be credited back to the credit card, the Pay with Miles portion of the fare will be returned as non-refundable credit equivalent to the dollar value of the portion of the ticket redeemed with Pay with Miles. Credit may be used to purchase future travel and Pay with Miles terms and conditions will not apply to such tickets.

In the event of a valid ticket cancellation under Delta's Risk Free Cancellation policy, Pay with Miles portion of the fare will be redeposited into the member's SkyMiles account and the paid portion of the fare will be credited back to the credit card.

Miscellaneous:
Any miles you use will be automatically and permanently deducted from your SkyMiles account at the time of the redemption. Except for mileage accrual, upgrades, and form of refund for refundable fares, all other fare rules for the ticket purchased apply. All SkyMiles program rules apply. To review the rules, please visit Membership Guide & Program Rules.