How to upgrade from Android 7 to 9, with Xiaomi Redmi Note 4(X) [mido]

I just knew that Android 9 (Pie) is now officially and stably supported on /e/, as it displays the website for searching devices: Smartphone Selector

Will this version upgrade come alongside with next /e/ upgrade (to v0.18) or will it be mandatory to reinstall the OS?

you mean /e/ android 9 Pie, i suppose, because it is the current grade for the official mido versions e-0.16-p and e-0.17-p.
first upgrade to xiaomi android 9 if possible, but not to 10

unofficials mido are on Q

I first backed up system and internal storage data, following these steps: [HOWTO] Backup and Restore Data on Android Devices
Then, I followed these ones to upgrade from Android 7 to 9: [HOWTO] Upgrade manually from e-Nougat | e-Oero to official e-Pie builds without data loss

All went without any problem, absolutely recommendable