Drupal-8.3.1-56 is available for customers however the clients using Drupal 7 cannot upgrade it to 8 version due to absence of corresponding option in CCP.
Upgrade of Drupal 7 to Drupal 8 is not possible because of the APS upgrade matches settings. According to metadata of package Drupal-8.3.1-56, upgrade to version 8.3.1 is possible only from versions 8.0 and higher:
<upgrade match="/application/version >= '8.0' and /application/release >= '1'"/>
Consider using internal Drupal methods to move the site to Drupal 8.