Can a travel agent get me a deal I can't get for myself online? Is it cheaper? Is it more expensive? Which one to use? What is the benefit in using an agent?
Disney has one price. This means every website or travel agent that sells a Disney vacation has to sell it for the same price. The flights can have some variation as that is considered separate so you’ll different pricing on packages that include flights. I focus on what is the best value for your vacation dollar. Is it really a better deal if you are 40 minutes from the Parks in two double beds if for $100 more you could have been two queen beds walking distance from Epcot? 

The benefit of using an agent is having someone who is in your corner helping you figure out the best options for your family, answering your questions, and providing support and assistance throughout the whole process. 

As to which one to use, I hope you’ll consider using me. 

Are there additional costs? If better deals come after I've booked, are you able to change/update reservations without additional cost or penalty?
I do not charge any fees. If the deals come out before your final payment, I can usually update your reservation without an additional costs or penalties. It just depends on the terms and conditions of the package you booked. 

Do you know from experience or does Disney give you a guide?
Both. Disney has a special training program for travel agents called College of Knowledge. I have graduated from that program. I also visit Disney World frequently so I have my own personal experience to use as well.