ETA: Never mind. Google had the answer:

• Delta eCredits are NOT eligible to be applied toward the purchase of any miscellaneous transactions such as upgrades, Delta Sky Club passes, baggage fees, UMNR, etc.

Someone can chime in as to whether this would be a way around the mileage transfer costs:

Perhaps you, the OP, can buy a main cabin fare for your cousin using your miles, being sure to input her Sky Miles number when you do it. Then, once the trip settles into her account, she may have a first class upgrade offer for cash that she could then use her E-credit for.

I don’t know if E-Credits can be applied to upgrade prices or Award Tickets. Someone else will likely pigeonhole this idea, but it would be interesting to see if it’s possible.

ETA: I could probably check this out for myself. Both my wife and I are DM‘s, and I could get her an award ticket for sometime in the future, then try to apply one of her E-credits toward an upgrade to first, and see if that works. If it doesn’t I can always do a redeposit with no ill effects. It’ll create a little bit of work potentially for Delta’s computers, but I don’t see how that could hurt unless it stains my reputation with Delta.