If I upgrade to Todoist Premium will I be able to then see all my past completed actions from the past year? Or is it the case that once you've upgraded you can only see the actions you've completed after the upgrade?
You can always (even for free) see your completed tasks if you enter a project and click on the circular icon in the bottom-right corner to load completed tasks.
With Todoist Premium, you'll have the option to see them all at once, filter the results, see your productivity on graphs and all this data will include each completion of all your tasks made in the past. So you will see everything, not just items completed after the purchase.