I had this exact same question a couple months ago. I had a free dining package with a 5 day ticket and wanted to add a park day but was scared that changing it would lose the package deal. I also didn't want to call if I could possibly do it online because I hate the long waits and convoluted phone system. Another DIS poster was able to reassure me that I could make the change online and it wouldn't affect the rest of the package.
So, I tried it. Total success! I think WDW makes it as easy as possible to spend MORE money. Good luck.