Windows Live Messenger started off its life as MSN Messenger way back in 1999. After almost 14 years of service, Microsoft will be retiring WLM in favor of Skype and users will be able to merge their Live Messenger account with their Skype account.
In earlier reports the rollover date given by Microsoft was March 15th, supposedly the last day users would have to make the transition. According to Skype's own blog, it now looks like the date is being pushed to April 8th. That's the day Skype will roll out the "upgrades" starting with English language clients. Last on the upgrade list are Brazilian Portuguese language clients estimated for April 30 or later. The only exception seems to be mainland China, where Messenger will continue to be available.
According to TechRadar desktop client users will get notified to upgrade and will be unable to use the service until they accept. Apparently the change is aimed at older desktop clients since, for unknown reasons, Messenger will continue to work with Windows 8 and Windows Phone 8.