First, check for any provisions in your service agreement, if any, dealing with the ending of your relationship with the plan, a merger, or other plan amendments that could impact your scope of services or fees. Look specifically for provisions for a deconversion. You may be pleasantly surprised that the service agreement spells out some terms and conditions that are favorable to you. The SA may also say explicitly services that you will not provide. This can provide you with an opportunity to charge extra for a service you are not obligated to provide but are willing to provide.
Also note that your SA may specify time frames for deliverables as part of the process. Don't feel obligated to make extraordinary efforts and jump through hoops for the convenience of the successor provider.
Next, try to get a copy of what the receiving plan's service providers have in their requirements documents regarding their assumptions about data to be provided by you (as @Lou S. lists in his post). If your service agreement does not commit you to provide what is being asked of you, you can refuse or negotiate what you are willing to provide.