I would like to replace a clip on the time line with its transcoded version but there is no such action. The replacement should also consider the replacement of the extracted audio track. It would have to check that the duration of the clips is equal and use the same part of the clip as before.
This would be also be EXTREMELY USEFUL for animation and VFX workflow. I am aware of Kdenlive internal "proxy" system, but this ignores an important other reason for using proxies: sometimes the reason is that the final version of the shot is not yet available. Routinely, we create GL previz versions of shots, which can then be used for editing. But replacing these shots with the final renders (or even updated GL renders) requires a laborious process of re-cutting the new shot to match the old, which is just stupidly wasteful, from a workflow perspective. It should be possible to simply drop in the replacement shot and update the clips that refer to it. Clearly some error handling would have to deal with cases where the clip doesn't match the specs of the original clip, but most of the time, it will in this case. In fact, we do have a trick, which is to actually load a SYMLINK to the shot, then update the symlink at the file-system level to the corrected shot. Kdenlive doesn't know about this, though, and it doesn't update thumbnails or other things it technically ought to do. But I'll think you'll agree this is an ugly workaround!
Replace clip is possible with Version 20.12.2. You can do that in the project bin. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)