Close Encounter is one of the most challenging wave in CI3, so I think it’s good to appear in CIU. While old ufo is kinda “outdated”, why not just replace it with UFO chick & chickens? They could pose as bigger threat since UFO chicks shoot in /\ pattern and UFO chickens target you and both drop UFO debris.
Harder variant will allow them come from both side.
Chicken Square Dancing, Chicken Geometry, Terminator Chickens, and other waves that make you poop in your pants no matter how good of a CI player you are.
What makes it challenging and unique is that you couldn’t easily shoot them because they shot downwards and the screen was flooded with them, so Ion Blaster and such weapons couldn’t easily take them down from an angle. It’s better to keep it in its original form, with retextured UFOs to match CI5’s style.