the new Release is only effecting new installs … we do only release a new installer fixing things and may adding options and stuff. Installed System is updated as usual on a rolling release and will be the same.
That`s also always mentioned in the release note:
The changes described over here are affecting new installs, our Calamares installer, and the Live environment on the ISO only. Running systems don’t have to “upgrade” to Mercury, if you update regularly your system is fine.
https://forum.endeavouros.com/t/our-new-release-is-here-meet-mercury