Not hard. Bit of cutting, but no necessity for re-framing in this case. Your window contractor can probably fix you up pretty simply and doing that as part of the siding project will eliminate any duplicity of labor.

Exactly the arch top rough openings are typically framed squared and the blocked in the corners for the arch.

Thank you both for your responses. I'm happy to hear that it shouldn't be too complicated. As for the style of the window, should I have grids inside of them? I've removed the grids to get the feel for it, and while it's cleaner, it feels a bit plain. I'm hitting a wall visualizing, as I have never seen the house anyy other way, so outside input is super helpful. Am I on the right track removing the arches, or am Im removing all of the character?

What would you do if this were your home? It definitely needs landscaping but am at a poss there as well. I am in Northern Illinois. Thanks!

I do agree that the arches look a bit dated. Some more landscaping, larger windows there (although I am not sure I would totally fill up that space as they are pretty big now).