Hi Ching,
Yes, you can change existing Disney resort reservations to reflect newly released promotions. You would just need to call Disney reservations or your travel agent to update your reservation. Therefore, it is a good idea to make reservations now and update them later as new offers are released. That way you won't miss out on reserving a room at the resort of your choice. Remember to check the
special offers page on the Walt Disney World website periodically to read about the latest promotions.
Regarding the other part of your question, can I assume since you are headed to Port Canaveral that you are setting sail on a Disney Cruise? If so, you might want to consider arranging your ground transfers from the airport to Port Canaveral, and from Port Canaveral to Walt Disney World through Disney Cruise Line. You can book these ground transfers by calling (800) 951-3532. Disney ground transfers are comfortable and incredibly convenient. I rode on a Disney bus from Disney's Polynesian Resort to Port Canaveral and from Port Canaveral to Orlando International Airport and both experiences were hassle-free and comfortable. This is the most cost effective way to go if your travel party is small.
With a family of three, however, I found that renting a car at the airport, driving to and from Port Canaveral, and then back to the airport was more cost effective. It all depends on prevailing rates for rental cars. If your group is large, you may want to consider this option. Private car services are an option, too, if you prefer not to drive.
Wishing you the best of luck as you plan your April 2012 Disney adventure. Let us know if you have any other questions with which we can assist you.
Jill