UPDATE: Geoff Goodman made another post that clarifies the current PTR changes.
Let me give some extra context for these changes.
First off, these changes will not be included with the other changes on the PTR, which will be coming out with the Halloween update. These Mercy changes are purely something that we're experimenting with, and there will likely be more changes coming to try out.
There was a bug where Mercy's Guardian Angel momentum wasn't slowed correctly sometimes when canceled. This was intensely frustrating for most Mercy players, but some players enjoyed trying to use this bug to gain extra mobility. This bug is important to fix so it doesn't feel like Mercy randomly gets out of your control, but we're looking seeing if there is a way to include some of what people enjoyed about this bug as new functionality for her.
To reiterate: These changes aren't hitting live any time soon. These are just things we want to try out for now, and they are likely to change.
Mercy’s recent rework has been a success. Maybe it’s been a little too much of a success! Overwatch players have gone from begging their teammates to please pick a healer, any healer, to fighting each other over who gets to play Mercy. Her recent rework removed her ability to ressurect every dead ally around her, but she can now resurrect one person on a 30 second cooldown as a basic ability, and her new ultimate resets that cooldown.
Blizzard are hitting that ability on the latest Public Test Realm in an attempt to decrease her power.
No longer resets or lowers Resurrection’s cooldown. Instead it now doubles Resurrection’s range.
With careful use of Valkyrie, players could rez an ally, hit Valkyrie, rez another ally, and then wait out the timer and get a third person up in just over thirty seconds. Obviously, that’s not nearly as powerful as her old ultimate... but it was an extremely powerful tool, especially since she can now fly during her ultimate.
Valkyrie will now allow Mercy more safety during a fight, so she can watch over her allies from a distance.
Of course, all changes on the PTR are subject to change, so we may see more tweaks to the new and improved Mercy in the days to come.