Checklist(s) for simplifying upgrades from Drupal 6 to 7 or higher in the future?
I have used the Upgrade status module on the past to better understand how much work will be involved in upgrading a site from a mayor version to the next.

Also, depending on how many modifications, or modules you have, it will also make the process easier. 

One more tip: Check the Drupal documentation, and remember Youtube is your friend.