Office 365: one or more users need an assigned license

I recently setup a five user P1 Email system on Office 365.  Iniitally I setup the default xxx.onmicrosoft.com domain and added the users.  

Then I added the "Vantiy domain" ie mycompany.com  (why Microsoft call this a vanity domain is beyond me, Id call it an essential to do business domain, but....)

I then changed the logon primary account for each user from @xxx.onmicrosoft.com to @mycompany.com - (There are a number of poweshell scipts around for this, however you can do it from the Office 365 Admin GUI.  Just go to the users and once the Vantity domain has been validated you can change the login domain to the one you have added.)

I then started receiving a typical Microsoft Cryptic error message: "one or more users need an assigned license in order to retain an exchange online mailbox or archive."

I created a view as they suggested showing unlicensed users, and it showed all my users...!

It took me ages to work out what Microsoft 365 were on about, but it seems that the license is assigned to the logon name account - and so changing it to mycompany.com domain confused it.  All other settings and data such as email etc stayed in tact.

This has got to be an Office 365 bug

Work around fix:

• You need to go to each user on the Admin screen on office 365,
• go to licensing
• un-check the Lync license
• Save and update
• Then go back into the account again
• check the Lync license
• Save and update

Hey presto problem has gone.  It would seem that re-applying a small change to the license, re-assigns it correctly to the changed user@mycompany.com

CAUTION DO NOT Uncheck the Exchange License as Ive seen dire warnings saying this will delete the users email mailbox which would be bad !