Due to the complexity of this project at the offset, I had already made a smart move and brought in to work with me on this project, my former colleague, friend and Drupal mentor Marci.
On the night of the migration, multiple backups were taken, databases were merged, bugs were resolved and the system was brought back up the next day working with the new upgrade in place.
Waiting for a call the next day to confirm everything was ok was a bit of a nail-biting experience. There was a minor issue with some block displays. Other than that, a really good result.