I grabbed dandelion greens at the store instead of mustard greens. Can I substitute or do I have to go back to the grocery store?

Gee, I find mustard greens more bitter than dandies. They're also less "hardy" -- as in they need much less cooking time before they're ready to eat, so I'd adjust the cooking time for them.

Certainly--in Portugal, the soup is made with a whole variety of wild (and cultivated) chicories. The amount of bitterness varies from one type to another, and from one season to another, but that's what adds interest to the recipe. But what do you do if the soup gets too bitter? Same thing you do with too much salt--add potatoes, cut in dice, and cooked in the stock. It will be a different soup but still, I'm sure, delicious.

I'd use half the amount of greens -- they'll be more bitter than mustard greens but I think it's worth trying.