When migrating an existing Drupal site, whether hosted externally or within Pantheon in a different Pantheon org, we will make a new site within the DIT hosting Pantheon org. This allows us to keep our environment and billing consistent. To request a DIT Drupal site on Pantheon, please use this link
The existing site to be migrated must be running the most recent version of Drupal 9, as all new sites created within the DIT Pantheon Drupal hosting are Drupal 9. To help upgrade a Drupal 8 site, we have the following KB article.
The new site will be a simple Drupal install, with a copy of the UMD Terp theme. It is not required to use the UMD Terp theme, it is provided as a resource. The existing site can then be copied into or on top of the base site, depending on the needs of your developers.