I'm sure at some point this month you've been in a scenario where you've killed an enemy during their ult, only for them to come back 30 seconds later with that ult somehow still prepped up and ready to go. Its not exactly a fun thing to experience, so I'm very glad to say that the next Overwatch update will finally be solving this age-long problem!
Under the new rules ultimate charge will be expended the very moment the player uses their ultimate, instead of being depleted over a short period of time. Heroes that are going to be most affected by these changes are Genji, Lucio, Soldier 76, Mei, Bastion, and Reinhardt. So if you're one of those people that love to use Lucio's ult from the very top of the map, it might be time to reconsider your approach, for more reasons than one.
That said, these changes aren't meant to be a nerf to any hero in specific, but they will be a noticeable decrease in power for Mei, Bastion, and Genji. Whereas most heroes can just press their ultimate button and start doing their thing, heroes with long animation times are going to be at a significant risk of their ultimates being essentially worthless. For example, even if you instantly interrupt Pharah she's still going to rain down a bit of damage, whereas stunning Genji or Mei during their overly long animation will make their ultimate disappear before it even does anything. Whether this is going to be a balance problem or just a minor annoyance, I can't exactly say, but its definitely something Blizzard might want to look into, especially considering how underplayed Mei is right now.
The patch will also bring with it a couple of bug fixes and minor tweaks to the 2CP game mode, so if you're interested in all of the details you'll find the patch notes over here. As for the release date, I'm afraid nothing has been said just yet, but if no further changes appear on the PTR you can expect to see it in the next week or so.