I was so nervous about that I asked 4 different times on subsequent calls for other things *LOL* So hopefully this will help ya.
They all told me that you don't have to change them- just be sure to tell them when you check in at the restaurant that you are on the plan and also tell your server too just in case. But you don't have to call and change ADR's that you made before you were on the dining plan, UNLESS! and this is a big unless LOL. Unless you were booked at Cindy's or something where they charge the whole amount up front to your credit card: cause then they would need to refund your card and put down that you were on the DDP.
Hope that helps! I like helping. I hate to think I'm asking all these questions and spending all that time listening to static-y hold music just for me! LOL
Laren