Ordinarally I'd say windows aren't structural but in this case I see why you ask. One window appears have joists resting on it and the other appears to only have a rotted beam between window and joists.

They are not structural and can be cut.

They are often cast in place and can be a bit more work then the appear.

Aren't structural like Todd said. Can cut it out, not easy, or put in a "buck frame" to create and opening that is continuous and ready for setting the new window.