@stormi I've upgraded the master in BIOS mode, and proceeded to reinstall one of the other hosts in UEFI mode, after having removed it from the pool. It went fine, until I tried to add the host back into the pool. This was not possible because the reinstalled host has Certificate Verification enabled, while the pool doesn't. Even if I host-emergency-disable-tls-verification, it's not possible to add the server back into the pool.
It seems I'll have to upgrade the other hosts in BIOS mode, enable certificate verification on the pool and then add this host. I guess I'll reinstall the other hosts in UEFI mode on a future upgrade.