Okay, this is how the changeover is going to happen:
* We're changing the site-wide default site scheme tomorrow. This will affect all logged-out users, and any users who create their accounts after tomorrow: their site scheme will be Horizon.
* Any existing users won't be affected tomorrow. Existing users who are using the site in Xcolibur will still see Xcolibur, at least for the few weeks after we change the site-wide default.
* Existing users who would like to keep Xcolibur as their site scheme will need to go to http://www.livejournal.com/manage/settings/ and explicitly re-save Xcolibur as their preference.
* After a few weeks, any users who are still using Xcolibur but haven't explicitly indicated Xcolibur during said same few weeks will be switched to Horizon. (The preference is always saved in the form of "I prefer to use this non-default scheme" vs "I would like the default scheme", and as long as Xcolibur is the default scheme, changing your preferences before we change the default site-wide won't have any effect.)
For more on why, see the lj_biz post.