How do you change a feature definition for a line you've already made?

Grace, that setting on the Feature Definition Toggle bar is not to 'change' the Feature Definition, it's to 'define' it 'prior' to layout. To change it, just use either the Quick Properties from the fly-out menu on that alignment, or the full Properties, or the Geometry > General Tools > Standards > Set Feature Definition tool.

What do you mean by the fly-out menu? I'm kind of new to ORD.

Selecting any ORD Civil object and pausing, will produce a fly-out menu. The first icon is the Quick Properties.

Grace, I STRONGLY recommend that you get into an OpenRoads Class, and procure a good reference book. ... or at the very least put yourself through all of the relevant ORD training available online. In my opinion, ORD is not something that you will just 'naturally' pick up without burning considerable time.

Oh, I know what you're talking about now. I've just never called it a fly-out menu. Anyway, I tried changing the feature definition using the properties button and the feature definition was grayed out. It wouldn't let me alter the text. Do you know why that is?

It looks like you’ve selected a 3D element from a 2D model. The software creates and attaches a 3D model to the 2D model as soon as any command is used that will require 3D information. This is expected behavior (and desirable) but you need to be aware of the workflows that go along with this. Generally, you should build your 2D elements and associate 3D information, such as a profile and let the software build the 3D element from that data. If you build something 3D in the 3D model, there is a tool to create a 2D element with a profile from a 3D element.