Blizzard have posted a heads up notice on Overwatch forums, warning players that the next patch will require a full reinstall of game files. Apparently, it has something to do with inability to patch over current content efficiently.
Blizzard are pulling a Bethesda on their players apparently, as Overwatch's next patch will require players to re-download and reinstall all game files when it kicks in. The official post didn't explicitly mention re-downloading game files but they did say that they "know this isn't ideal for people on metered connections" which implies all the files will have to be wiped and re-downloaded.
According to the post, this wasn't a decision Blizzard was looking forward to, but the next patch is "substantial enough" and bears "pretty fundamental changes" to the game client, which resulted in it being impossible to deliver over the current content with no issues.
Blizzard ended on a note hoping that this heads-up will help players out with preparing for the big download. Then again, it can't really help people with metered connections as it will likely mess up their bandwidth limit for the month regardless.
On the bright side of things, players will not need to re-download 50 GB like they did in Fallout 76, as Overwatch is rather light for today's standards, taking up just 13 GB of space. Bear in mind that there is currently no official statement on how big the actual update will be so it's possible that the total file size may go a gig up or down.
There is currently no ETA on this patch but it will presumably be the one when Ashe pops up. She has been recently added to PTR and the patch will be tested throughout the next week. Some players have already started speculating on what the big patch may entail, but Blizzard pretty much revealed this will not be the case in a statement to Kotaku.
According to the mail, the patch will be a remaster of sorts and will bring better loading times, disk space optimisation and bug fixes. Players who already have the PTR version of Overwatch will have a "significantly smaller" download when the patch lands.