Make a variant the asset's default.

The variant's pixels replace what the asset serves, at the SAME URL, so anything already embedding that URL shows the new file. The previous default is preserved as a new variant and keeps the name it had; the asset keeps its id and URL and takes the promoted variant's name. Its description and the tags generated from its image follow the picture — the promoted variant hands over any it carries, and the preserved variant keeps the ones that described it, while tags you added yourself stay on the asset. Rejected while the asset is a video that is still being processed. The promote is accepted and the swap completes a moment later; re-fetch the asset to see it reflected.