Is it possible to change the compatibility mode for a new sandbox to a previous version on the B2C Commerce platform?

If there is a file called .apiversion within the code version, then the customer can go back after activation of that code version. If no .apiversion file exists in the customer's code, then they can create one based on the format below, and upload it with their code base. Once the customer activates their code, the compatibility mode will be updated to match the version in the .apiversion file.

Please note that the file .apiversion is required to be uploaded to the WebDAV "Code Version folder" (/webdav/Sites/Cartridges). 

A generated .apiversion files looks like below - property api.version=<API Version> should be the content of the file.

#DO NOT EDIT! This is the api version with which this code version is compatible. It is managed by the system.
#Wed Nov 06 19:13:39 GMT 2019
api.version=18.2

Please do not edit the .apiversion file on any instance other than a sandbox!
 

For non-sandbox instances, please see Changing the B2C Commerce code version compatibility mode to an older version.