Are you running it as Admin?
You may want to try duplicating the current installation then renaming one folder to Elder Scrolls Online (EU) and try the Repair function if it doesn't work.
I can not seriously believe they are going down the path of requiring players to have two separate 30GB+ installation folders just to be able to play on EU and US megaservers. For the Mac version it's over 65GB of HD space if that's that case. And close to 100GB HD space for the install process if you already have one folder installed.
It may well be possible to simply rename the folder as it was before but who would want to do something so clunky each time they want to play on the other megaserver??
There can only be a moderate size difference if they are aiming to have the language support separate and I can't see at all the need for a entirely separate folder when the vast bulk of the install is the .dat data files which would be shared be both anyhow. If WoW can support even more language options on one Installer it can't be needed,
Only thing is that they are not syncing character progression over the two megaserver which is fine. I get that. But you can't use the same character on both in any event come live launch.
Submitted a ticket to clarify this.
I would definitely make a backup of the current install in any event to save fresh reinstalls given the fact it takes 30 hours+to download the beast as it is