Just installed KDE version, then did an upgrade, and got a number if signature errors like this one:
error: python-idna: signature from “Chih-Hsuan Yen yan12125@archlinux.org” is unknown trust
:: File /var/cache/pacman/pkg/python-idna-3.7-1-any.pkg.tar.zst is corrupted (invalid or corrupted package (PG
P signature)).
Do you want to delete it? [Y/n]
If I delete them, I get:
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.
to update only the keyrings first. After that, the normal sudo pacman -Syu worked without problems. In fact, since all the packages to be upgaded had already been downloaded earlier (before the update aborted with the error message that you mentioned), the update completed pretty quickly.
Hope this works for you, and that you don’t need to do a full reinstall.