A partial release of the update happened last week but Microsoft cleared it out that they plan to go slow with the update to ensure a stable release.
Graeme Boyd, Xbox EMEA social marketing manager tweeted regarding the news:
I hear that the Xbox LIVE dashboard update should be going live now for all, enjoy!
Boyd also cleared why some Xbox Live subscribers got the update in advanced.
It was based on a number of factors including location, ISP, bandwidth etc. It was a stress test, basically.
Enjoy rummaging through your new dashboard once it’s updated.