Hi Macjett,
I had the same issue this evening as downloaded Mac OS Sierra this afternoon all went well everything went across all good but had issue with iMovie 10.1.3 after installed then there was a new release one I re-installed that straight after installing 5 apps with the new iMovie maybe that had something to do with it.
I followed your step by step what you wrote and deleted the iMovie into trash as haven't yet removed from trash completely i even went to update again in Applications as still current now updating but think should be fine as I opened up and came up as welcome then clicked on prompts and there was our old videos from Lion iMovie as hadn't done no upgrades from Lion due to some reviews on Yosemite & on El Capitain. Even contacted Apple to ensure was right to go ahead without previous installments. Sent email to Apple to call them tomorrow about iMovie 10.1.3 but don't think need to now following your instructions. I'll keep old one for another day in trash or re-check now again before emptying as kept incase needed if had to ring Apple. Thankyou so much helped me alot & deleting the folder from Containers in Library etc.