We can hope that they'll be getting rid of the PWA mess that they've had going for the last few years. Realistically speaking however, the bugs will probably just be swapped for new bugs.
From a technical perspective, usually that kind of window isn't necessary, though I've seen exceptions, mainly in oldschool monoliths that can only be configured on their very specific production hardware (I assume this isn't what's happening at Uniqlo). I've worked with large companies that are fine with eating those kinds of losses if it means reliability for the launch. One client, prior to launching an upgraded version of their site, asked us "is two weeks enough time to set things up?"... we proposed a few hours of downtime, but they were prepared and comfortable to remain down for that window.
