Hi Claire,
According to the Terms and Conditions on the Walt Disney Travel Company (WDTC) website, for holidays which include accommodation but do not include flights and / or a cruise:-
If you make your booking more than eight weeks before the intended departure date, you must pay a deposit of £50 per person, which is payable at the time of making your booking. The balance of the cost of your booking becomes payable in full eight weeks before your departure. Payment may be made by visiting www.disneyworld.co.uk/payment or calling 0800 169 0730;
If you make your booking eight weeks or less before the intended departure date, then the total price is payable in full at the time of making your booking.
You can read all about what would happen if you change your booking by checking
Section 10 of the Terms & Conditions.
Likewise, you can read all about what would happen if you cancelled your booking by checking Section 11 of the Terms & Conditions. In a nutshell, if you cancelled your booking more than 56 days prior to your departure date, you would lose the deposit of £50 per person which was paid at the time of booking.
If you are unsure about anything, I would highly recommend that you give the WDTC a call on 0800 16 90 730 or send an e-mail to wdtc-uk@disney.com.
If I can be of any further help please don't hesitate to ask me another question.
Wishing you a magical trip filled with memories which will last a lifetime!