Words from the mouth of Twitter are "You will automatically update to latest Twitter very soon".
As most of users didn’t change to new version and continued with friendly old version must have seen a yellow color banner stating to users to switch to new version.
Twitter from now will not ask users to switch as they have implemented new version to all users. This will be happening very soon. Twitter did announce the release of this latest interface for Twitter about nine months ago. Conversely, bundle of twitter users must be thinking when this is going to happen.
There are many technical difficulties that the latest twitter have, as per some complains from the latest twitter version users. Twitter will first rectify all the technical snags and difficulties and then will make complete transformation to new and latest version of Twitter for all users. After twitter transform to latest version and did find any new or old technical and or other difficulties cropping up then website will shift to old original version when the latest version feels the heat of problems or some bad code.