It's no surprise that Windows Server 2012 R2 will be coming out on 18 October, essentially the same date as Windows 8.1, since the two products have been on a parallel development cycle. But while I can imagine plenty of individual Windows users moving to Windows 8.1 on the day of release, that's less likely to apply to the server versions.
The preview version of Windows Server 2012 R2 has been available for download and testing since late June, so four months will have passed by the time it officially appears (along with System Center 2012 R2 and an update to Windows Intune). While that's a reasonable period of time for testing, Windows Server 2012 itself has only been out for a year. In workplaces where it has only just bedded down, switching to a new version is unlikely to be an appealing option, unless one of its new features is utterly crucial.
Will you be switching speedily to Windows Server 2012 R2? Tell us in the comments.