Unfortunately, that means you have no choice but to find a way to make it work if you have a capped connection. Blizzard doesn't have a release date for the update yet -- the company only issued an early notice, most likely because some users might raise hell if they suddenly have to reinstall a whole game when they were merely expecting a patch. Once the gaming giant announces a date and time, you may want to plan accordingly and schedule your other downloads for a later date.
Here's the company's announcement in full:
"We wanted to get ahead of this early and let you all know what to expect with our next big patch. We don't currently have a release date/time with this, but will update this thread once we do.
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. 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.
Thanks for your patience with this matter, and we'll see you all in game!"