It appears that the Messenger team is planning to introduce upgrade notes to all Messenger users soon. From today, 17 February 2009, onwards, the first notices will be appearing in the Messenger 2009 Beta version. Additionally, all users using the Messenger RC build (14.0.8050.1202) are asked to upgrade their Messenger version to the latest QFE build, just as previously announced (without telling any dates).
The next step will be upgrade notes appearing on the Messenger versions 8.1 and 8.5. Users of these versions will start to see notes from 25 Febuary 2009 onwards. The last bit will be started from mid of April, when all Messenger users see the upgrade notices on each sign-in. Microsoft is recommending to install the latest version because else some Messenger functions may not be working. For users of Messenger 2009 Beta (14.0.5027.0908), the upgrade is mandatory.