New Salesforce Release. Preview sandbox vs non preview sandbox.
Three times per year Salesforce.com launches their feature releases. These releases contain lots of new features and changes. Before any production org is updated to the new release, Salesforce allows you to upgrade the sandboxes you chose to the new release (Preview sandbox).
There shouldn't be any problems when using Copado with preview sandboxes and non-preview sandboxes. Copado will use the old API version until Salesforce upgrades all the environments to the new API version.
Now, according to Salesforce best practices, you should never deploy from a preview sandbox to a current release version. This is applicable to all environments, not only production. Since the newer version has a new code base, changes in methods, Apex handling, flow automation and many other areas, Copado might be affected by the changes introduced by Salesforce in the new release.