It's a little bit too late isn't it?
So an announcemnet of update
6 hours after the deed is done.
Is this update not planned??
And what do you mean you can't say WHEN the update start so people can get ready? I mean the only possibility I can think of is the update team got into late night party and got a hangover so to update it they need to be sober first. If not then why can't you say when it's gonna start???
In my opinion though it's better if you just say 'today there was an attack on our server and tomorrow we'll see to/fix it'. Noone can't predict when an attack is gonna happen, but an update without planning?????!!!!
Just say that at 01:00 UTC server is down for indefinite time for maintenance/update. That will reduce people grieves somewhat.