I upgraded my U790 panels to U890 bass focus in 2018.
As the panels are taller than the original frames, as they are with the U1’s your contemplating purchasing, I had SL make a set of extensions so the speaker looked finished. The extensions came to me unfinished so I could have them painted to match the color of the existing frames by a local auto body shop. To the rear of the frame, rectangular pieces of kydex/lexan were added to match the existing mounting flange and finish the look.
See the attached pix. and diagram to see how it was done.