Can I install a new gas fireplace without changing cornices?
Your choices will be limited by what your local regulations say about the options for the type and location of flue and ventilation air supply. A reputable local supplier should be able to advise on the technical and visual aspects.

You might be allowed to put a flue behind the fire on the outside of the wall. In the UK we can get shallow depth gas fires and surrounds for flush mounting onto a wall. This avoids having to build a chimney breast to accommodate the fire depth and route the flue through the ceiling internally.

I think a ceiling height chimney breast would look okay and you could blend in some similar cornice to follow the new outline. The installer might need to cut through and remove some of the existing cornice hidden inside the chimney breast where the flue goes through the ceiling.

I think having the chimney breast stop part way up the wall would look odd.

If the fireplace is mainly for aesthetic reasons, a decorative electric fire and surround might give you the same benefits but without the problem of having to construct a flue. Again check for local regulations and also the available power output of the outlets.