Correct. Until T-24 when DL gets control you're basically on a KLM FLIGHT since it's coded as KL. Within 24hrs control switches to DL who then operates under the DL FLIGHT number.
Basically a codeshare is a carriers flight that they know they won't operate so everything is sent over to the operating carrier to reserve space on their flight that operates at the same time. However the operating carrier can't process cabin changes till T-24. Otherwise the KLM and DL system won't be in sync and KLM being the marketing carrier would prevail. (resulting in downgrade)