I called the Disney tickets line with this question a few months ago, and they said yes, it is possible, as long as you're going to a more expensive ticket. The rep actually offered to do the conversion right then, over the phone, so I had to explain that I didn't have the tickets yet.
I second everyone's comfy pajamas idea. Shower tablets are also great for new moms - they're like bath bombs, but they go on the shower floor and release a nice scent. The first few weeks after a baby, showers are the GREATEST, and these make them even better. If you're thinking of nursing, nursing tanks are another idea, although not everyone likes them. They make lots of different shirts nursing-friendly so you don't have to buy a whole new wardrobe.