Mews understands rooms classification via the category Ordering.
You can find it in Settings > Services > Bookable services > Choose respective service (e.g. Stay) > Space categories.
The ordering allows the system to understand which rooms will be considered above or below others. This is especially important for upgrades and downgrades.
Mews PMS doesn't allow downgrades from the Timeline (drag-and-drop feature), but it allows upgrades in the same way.
You will need to Downgrade from the Properties tab of the booking.
Equal categories are considered to be at the same level.
When you input the same number in the ordering for different categories, the system still considers it a downgrade.