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As it looks right now I would have to reupload them every four weeks or so. I can imagine, that would upset all viewers who have already seen the old video. And of course... over time, It would add up to a lot of videos that have to be reposted.

Could you consider providing a YouTube link in the description, which viewers could take in case the video isn't running anymore?

It is not possible to add a link to a video that is more then seven days old. After seven days a blog-entry (surrounding the video) is written into the blockchain and can not be edited.

So its not possible for old videos.

It could be done for all new videos of course, but than we would have an embedded YouTube video under a D.Tube Video.

Most people would not understand.

sadly you are not able to edit older Uploads, so you can reuploud the Video via IPFS but you cant enter the new hash values into the original D Tube entry

If its the same file, its the same hash. Now, that might be different for whatever file the dtube uploader produces, but they dont use the uploader...