I had this exact same thought, but if you cancel within 24h, I heard it goes back to original gift cards.

I found a roundabout solution. To give context, I contacted Delta asking for them to help book a flight using 3 gift cards ($250) and 1 ecredit ($50). After talking to a supervisor, they said it could not be done. I also asked if the gift card could be converted to ecredit, and the rep and supervisor also said it could not be done.

So I came up with another idea.

1) Book a placeholder flight for as close to the value of my gift cards as possible. I found one for $255, so I used my $250 gift card and $5 cash.
2) Immediately click "modify flight," and change the flight to the itinerary I actually want to book (valued at $315)
3) The system allows me to use the value of my original ticket from step 1 ($255) against the cost of the new flight, and also allows me to apply any ecredit I may have ($50)
4) Pay cash for the difference of $10

This is the most ridiculous, roundabout way to be able to use gift card and ecredit together. I also don't know whether this would be even possible outside of times of COVID-19 with waived change fees. It is important to note that I modified my flight immediately within the 24h policy, but that due to COVID-19, any change fees would have been waived regardless.