No concerns at all. This is a very common thing to change.

No issue at all, and if any wires are shorter than needed you can extend them with pigtails.

Pigtails do not count in box fill, but that does not mean they may not make the box harder to work in or a tight fit.

Most of the time the pigtails are held as short as possible to avoid crowding the box and use the smallest wire nuts allowed for the wire size.

It can get to be a real problem to push everything into the box sometimes, especially if there are any dimmers present.

Dimmers while much larger than snap switches, still incur the same volume limit in boxes.