It really depends on the vendor and if they are available on the new date. I would imagine most vendors would be flexible and move the date (photographers that are very familiar with Disney weddings knows this happens and have no problems making the changes.)
I would be careful about booking vendors too soon and putting deposits. You change your mind SO much during the process of planning a wedding and if someone you like better comes along you then have the choice of booking the new vendor and losing your deposit or being stuck with what you already have. You have to be VERY certain those are the vendors you want. For example we haven't seen our video yet so I can't say if it was worth it but when we got closer we had spent so much money that we were re thinking our video choice (which we paid almost 5k for) and just going with someone else that had a cheaper package but we had put 50% down and no way would it have saved us money to switch. Also, if you find you do not click with that vendor you are in the same situation where you are stuck with them or lose your deposit.
Just food for thought. HTH