The Overwatch site has been updated with a new post to let the community know that the game's next major update will require players to reinstall the game completely. According to the post, this is due to the addition of "some pretty fundamental changes to the game client" on the back end. The post goes on to say that the "substantial number" of changes means that it isn't efficient to simply patch over the existing client.
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.
There is no ETA for when the next update will be released, but we'll keep you posted.
You can read the original post on the Overwatch tech support forum.