Google has announced that it has beta testing for its new version of Messages. Within 24 hours of announcing the maximum number of people joined the beta testing program. It's not the first time Google has opened up beta testing for its new features.
The Open Beta gives users an opportunity to test and provide their input about the experience to Google. The formal announcement about the Open beta comes from Google via an official post. Google often offers several Beta testing for its various apps that help Google to incorporate the features users points out through suggestions.
There is currently no official update about when the Messages will be updated for all the Android users. Google will first identify the bugs through its Beta testing and then will release a stable update for all users. Beta testing provides users an opportunity to use new features before the majority of other people.
One can remove themselves from the beta testing anytime they want to by visiting the same page they enrolled themselves on. There will be some experimental features Google has put in the Beta version that will make their way to the official app once Beta testing is done.