I have not had luck with making a reservation and then transferring into an interline rate. First, you must understand that these rates are limited to a particular group (have to show ID or risk being charged full fare at the port). Secondly, not all, in fact, very few, agencies deal with interline rates. I don't believe DU does. I was not aware that
DCL deals with these rates directly. This is a change from a few years ago when they told me I had to use an interline agency.
If you have a reservation with DU or any TA, it is bad form, if not impossible to transfer it to another agency.
When I have inquired in the past about booking a regular rate with an interline agency (they will do that) and the hoping for an interline rate to come out, I believe I was told that in order to get the interline rate, I would have to cancel and then rebook: read: lose deposit or more due to the late release of the rate; potentially lose the room I have selected and lose the dining time I have requested. Maybe someone else has had a different experience with this?
I have been able to get a price adjustment (on another line) when the interline rate went down. Don't know if DCL will do this with this rate like they do with others.