The same process actually applies when making an old DTube video playable again. What you have to do is to change the hash values generated by IPFS (which is the protocol that DTube runs on), for any resolution. Whenever a video (or a thumbnail/sprite) is requested afterwards, instead of requesting files with the old hash which may be gone from the network, it requests the file with the new hash from the network.
This is not the ideal solution you want to go for to keep old videos playable as each time you update old videos (you have to do this frequently!), it will use a lot of bandwidth. Eventually you will run out of bandwidth on your account, and can only be remedied by powering up more and more over time, which can be costly when STEEM price is high. The real solution to this is to host the content (videos + thumbnail + sprites) yourself, which is a topic for another day. 😃