Hi, Theresa! Thanks for asking your question here on the Disney Parks Moms Panel.
Yes, you can use your Table Service Credits for Quick Service Credits on the Disney Dining Plan. If you are only interested in Quick Service Dining options, I suggest looking at purchasing the
Quick Service Dining Plan instead. If you are not going to be dining at Table Service Meals, the standard Disney Dining Plan wouldn't be worth the cost for you. The Quick Service Dining Plan gives you 2 Quick Service Credits per night stayed at a Walt Disney World Resort and 1 Snack Credit. You also get 1 refillable mug, per person, to use for the length of your stay at the Resort. I think this would be a better option for your next Walt Disney World vacation.
Whatever you decided, I hope you have a magical vacation. If you have any other questions, please don't hesitate to ask.