I have changed a couple of times since booking with free dining, I checked first and was told I was locked in at free dining, you don’t pay anything if the cost is more than your original booking. The rate you paid for the park tickets is locked in as well (now they are slightly cheaper). However when I eventually found the OKW room I wanted, I rang and the cost of the room with free dining was a lot less than what was showing on the system that day without free dining.
I was also advised you pay £50 regardless of how many changes you make, I needed to change dates slightly, resort which was cheaper and the names of guests as well so I just made all the changes in one call for £50.