A few days ago, sign-ups for participation in the MIUI 6’s beta tests started. Unfortunately, only the China Unicom and China Telecom versions of the Mi 3 and Mi 4 will be used. Many users flooded MIUI’s forums and Weibo account complaining that they “abandoning” their old users by not including Mi 2/2S as well as the Mi 3 China Mobile Edition. Some even went as far as accusing Xiaomi of not letting the Mi 2/2S and Mi 3 China Mobile Edition support the MIUI 6.
In response to these claims, MIUI published a statement in their forums saying that they will not test the Mi 2/2S first due to having a limited number of developers and they will gradually do beta tests using other phones as well. In other words, the OS of the Mi 2/2S could definitely be upgraded to MIUI 6. It is just a matter of time.
Furthermore, MIUI said that they will announce plans to test other phones on August 16. [source]