While you can always upgrade an OpenStack instance to a larger variant, it is not technically possible to downgrade an instance to a smaller size.
The reason for this is that in a downgrade, you would also reduce the disk space. Since the data on your instance is written across the disk space and we cannot see which sectors of the disk are in use or not, removing a portion of the disk space would likely lead to data corruption or data loss. This is regardless of whether the additional space is already partitioned.
If you expect to switch frequently between smaller and larger Flavors, we recommend using a flavor other than the Standard one. With this, the disk remains the same when resizing the Instance. For an overview of the different flavors, please refer to our pricing overview.
If you still want a smaller instance, you can create a new instance and transfer all data from the current VPS to the new, smaller instance. Once all data has been moved, you can delete the current instance in the Horizon web interface.