IMHO, the only reason to use a
travel agent is so they can monitor for room deals or
free dining. I would never let them make fastpass or dining reservations. I've never heard of one that does touring plans for you. It's all easy to do yourself once you set up a My Disney Experience account through the WDW web site.
So, I would not normally use a travel agent. I'd rather keep control of everything myself. Did you have a travel agent in mind? The sponsors of this web site run a
travel agency specializing in Disney.
I think , once you book through an agent, any changes to the reservation must be made through an TA must be made through them and not directly through Disney.