Hi Kristine!
You can get cheaper rates during the off-season. The off-season is generally September-March, with the exception of holiday periods, where you'll see a bump in price.
Yes, you can change your trip if needed, if you find a better deal. Disney will most likely need to re-package your vacation.
What I do sometimes is go online and "act" as if I'm booking a vacation, just to compare the costs (I don't go the final step of actually booking the vacation or putting in my payment information :-) This gives me an apples-to-apples comparison of rates during particular times.
I also keep an eye out on the Special Offers section, which you'll find offers begin to pick up particularly for the off-season periods. To check the Special Offers page, click here.
Good Luck!