mariadb-upgrade-service.exe is currently not tested prior to releases. It is broken (as in completely disfunctional), currently due to CONC-760, and it was similarly broken before in MDEV-30639

While the intention is to use it for cross-version upgrades on Windows, neither old nor new buildbot had integrated upgrade scenarios, with 2 major version side-by-side. where a service instance is migrated from 10.x->11.x

First step to avoid regression, would be just to tests mariadb-upgrade-service.exe using windows service, to emulate "upgrade" from/to the same version, in MTR

What the utility does is service stop, start mariadbd for upgrade, run mariadb-upgrade, change service definition to point to mariadbd.exe with different full path, restart service

All of that can be tried with single version only, "path to different mariadbd.exe in service config" will actually be path to the same mysqld.exe

With even such a trivial test is in place, we could have caught both CONC-760 and MDEV-30639

A slight challenge is that the utility assumes that mysqld.exe, mysql_upgrade.exe and mariadbd.exe all reside in the same directory, which is true for installations, but currently the "build" layout differs from that.

Update: according to a comment in MDEV-19430, this scenario of cross-version upgrades is allegedly tested on enterprise builds by the CI, but it is not and never was in community server.