Yes, the amount of versions available from the Changes tab can be adjusted from the Document Versioning settings in CMS.
  1. In CMS, navigate to Reports and Tools > All Reports and Tools.
  2. Under Site Management, select Document Versioning.
  3. Use the drop down to select the amount of versions you want to store for web pages, images, files & XML documents.
  4. Save your changes.
Currently, there is not a way to revert back to a version beyond the 25 version limit. If you need to recover lost data in CMS, follow these instructions for recovering deleted content.