Use Content Manager to Change Content Database

Details:

To use Content Manager to move a site or site collection within the same Web application and change the Content Database it is stored in:

1. First, create a plan that will export the Site Collection to staging.
2. Once the content has been exported to staging, create a new plan that will Import from staging. 
3. After Selecting the appropriate Web Application, select the option to make a new site collection as seen in the screenshot above. Then select configure to select the appropriate Content Database.