The next major Overwatch update will require a full reinstallation

Blizzard has posted a notice warning that the next major Overwatch patch will be a big one—so big that it will require a full reinstall of the game. 

"The next major patch will have some pretty fundamental changes to the game client. The number of back-end changes we’re putting in is substantial enough that we can’t just patch over the current content efficiently," customer support rep Drakultoh wrote.   

"As a result, when the next patch comes out, the client will fully reinstall itself to handle these changes. We know that this isn’t ideal for people on metered connections, so hopefully this announcement helps you prepare for this coming update." 

I'm not sure exactly how a person would prepare for such a thing—download less, I suppose, maybe call a friend with an uncapped connection, or start scoping out local libraries to see if there's a way to sneak your rig inside and leech off their wifi—but for whatever it's worth, you've got a heads-up on the situation. A release window for the patch hasn't been set.