Do you know what part(s) of your outbound OGG-LAX-DTW-ATL-SJU were at medium level? If one segment is in medium, it will cause them all to price at medium. So if the already flow portion of OGG-LAX or LAX-DTW was priced medium, it should cause DTW-ATL-SJU to price at medium. But I'm not sure how it works with already flown legs and making the changes mid-trip.

Someone here can probably tell you if the change is even legal. You can't make changes to the trip less than 72 hours before departure, but you can make changes to the return portion. Even though you have a stopover, DL might consider OGG-SJU your outbound and not allow you to make any changes to the entire outbound.

As a start, just call up and ask them to price the change for you, should only take a few minutes.