The good news is that there is no shortage of builders making premium ukuleles-- in addition to the ones listed by DownUpDave above, I would add Maui Music (Peter Lieberman), Barron River (Allen McFarlen), Compass Rose (Rick Turner), Eric DeVine, Jason Wolverton, Dave Talsma, and Pohaku (Peter Hurney).
In my own experience, I own or have owned ukes by Luis Feu de Mesquita, Chuck Moore, Compass Rose, Maui Music, Mya Moe, Pohaku, and a custom-built uke by Dave Talsma. I also had the opportunity to play a John Kinnard uke for a week during a mail-around he did a few years back. Each and every one of these was an excellent instrument, but they were all quite different.
What does it mean to upgrade from a Kamaka? A Kamaka is a very high quality instrument. What is the goal? Better tone? I suspect you will get different tone from another builder, but better is subjective. A prettier uke? Nothing wrong with that, although my Mya-Moe was one of the plainest-looking ukes I've ever owned. Magical fairy dust? I've heard it happens
I will say this: despite having owned some wonderful ukes, my Dave Talsma uke is my prized possession, because I worked closely with Dave on the design, followed the build closely, and even had a couple of my own ideas incorporated into the design (or at least Dave made me feel like they were my ideas
). Not only is it a very fine musical instrument, it's a constant reminder of the thrill of being part of the creative process and something that was built just for me.