TL;DR when upgrading to Ultimate, if you select providers from chaotic-aur, it asks to replace some packages, which leads to end up the Setup-Assistant just after removing those packages without starting the upgrade.
I had a (not a big) problem while upgrading to Ultimate. After selecting packages, in Alacritty, when it asked to chose providers for some packages, I selected the chaotic-aur ones for each of them. Then, it asked that some packages have to be replaced, and I chose yes to remove the existing ones and replace them. Then, it removed the packages and that's all, Alacritty closed. Once again, I opened the Setup-Assistant and then went through selecting the packages again. This time it started downloading the packages.