Cocoa Autolayout - Why can't I delete or modify the (purple) width constraint on a Text Field?

I'm building a simple application using autolayout, and I've run into a strange situation. I place a Text Field in an empty part of a large open view so it's not affected by anything but the super view, but when I try to modify the "Width" constraint to be >= instead of ==, it creates a new constraint and refuses to modify the old one. I can't delete it, or change any of its attributes, because it just creates a new one.

Here is a comparison of the two constraints, the purple one being the stubborn one, and the blue one being the newly created one.

Why is the purple rounded one not modifiable?