Wow! Talk about a professional whose unsupportive. I'm sorry to hear that. I would check out Jack Newmans Breastfeeding site. He talks about lots of issues including unsupportive professionals. I'm sure all the information regarding toddler nutrition is included.
My personal thoughts:
if our breastmilk is designed to change with our babies need that it has changed for the better, not worse. If our milk is bad for a toddler whose body is not as sensitive to foods for reactions and such, than what does it do for an new born infant? How can something that feels so right, be so wrong?
Good luck, and follow your heart. No one knows whats better for your toddler than you!