If the advertisers are using AdWords, it shouldn't matter. Google automatically converts flash to HTML5 for those ads. If they aren't using AdWords, it would clearly be in the advertisers best interests to switch to HTML5 immediately.
You are most likely to notice which advertisers haven't switched because that their ads will simply be static images. Many, many advertisers still use Flash because, (1) it's easy, and (2) they are used to it.
As far as I know, Google will not be blocking Flash ads completely, only auto-play ads. Frankly, this could be a blessing in disguise for advertisers because I know of few things most people like less than an intrusive auto-playing ad on a website. Personally, I make a mental note not to buy anything from companies that have those ads, but I'm exceptionally vindictive and may not be representative of the wider population. :)