All bookings can only be transferred to a TA within the first 30 days of bookings. Also, note that the commission paid to the TA by
DCL is lower when you transfer than with an initial booking, therefore your OBC offered by a TA will be indeed lower if you transfer than if you book directly.
My understanding is that (at least with DU) you basically get half the OBC if you transfer (again, within 30 days) than if you name the TA directly when you OBB, or if you book directly with them.
Some other agencies that have a fixed priced might not have this issue. I know it's not a problem with
AAA since their offer is just a photo package. Not sure about Costco.
For OBB, you must name the TA at time of booking or you risk loosing some of the OBC, but you still must transfer it within 30 days.
For regular bookings, you basically need to book again with a TA to maximize the OBC. You could transfer it, but it would only make sense if the rates have already gone up. If you instead make a new reservation with a TA, you could work with DCL to release the particular stateroom to the new reservation.