Can you enable turnitin to go back to previous assignments that have already been graded?
My experience is that you have to have enabled Turnitin on an assignment before there are any student submissions. Therefore, I would be willing to venture that it is not possible to go back to previous assignments that have already been graded and enable Turnitin. If that doesn't help, you might find the comments thread on Canvas and Turnitin interesting if you haven't seen it already.
Claire, Travis N. Thurston is correct in that one cannot enable Turnitin reviews on an assignment retroactively after students have submitted. My experience with Turnitin is entirely with the Turnitin API (aka "the Legacy API"), not with the Turnitin LTI, so I can't say for sure if the latter allows a retroactive workflow. I'm going to tag Dallas E Hulsey, author and administrator of the aforementioned Canvas and Turnitin blog, to see if he can give you pointers on how to manage this.
Hey, Claire! If you are using the LTI version of Turnitin, then, yes, it is doable. If you are using the older API version, it is not.
Using the LTI version, here is how:
Retrieving data ...