Hi there - itch tells us "participants are free to update their submission page design and content while their files are locked."
Could you let us know if "page design and content" includes the option to add a playthrough video? If so, then uploading a playthrough video is totally fine with us, but if not, then you could only change the files after the voting phase ends. We are not exactly sure of how itch sets it up here. We have selected "Lock Submissions" on our end so that folks won't be able to update their files during the voting period.
Thank you!