These changes will occur in the next couple months but we are announcing them now to give content publishers and developers using HTML <video> an opportunity to make any necessary changes to their sites.

It's a ridiculous notion to expect all content publishers that have encoded their video in h.264 to re-encode their content for WebM just to appease Google. On top of that, they expect it to happen in only a few months' time! I am wondering whether YouTube is internally freaking out over this announcement right now: how long will it take them to re-encode all those videos?