This change has been flagged up for months and most people were changed ages ago. Its only those who manually reverted to the old version who have finally been forced to change in the last few days
I've a feeling the last few may have been changed across last night - when GMail crashed and 20% of users lost the service for several hours
I've noticed that mine changed to the new version and the option to set it back is now missing
The change is independent of browser