When moving from Lync On-prem to Lync Online there are like three or more different approaches, last week I tried one of them where my customer had lost all their Lync servers in a SAN failure.
So the mission was to get them on boarded to Lync online!
Well its just to create a tenant, add the domain, enable dirsync and it should work, right?
Well there is actually one more step to take.
When a user is enabled for Lync some attributes gets tattooed in AD, like the msRTCSIP-UserEnabled & msRTCSIP-PrimaryUserAddress for example and since these attributes has values in the on-prem AD, Lync Online will not provision these users and they will not be able to logon.
You can verify this by going to the users tab in the Lync Admin Center in the Office 365 portal. You should then see all the users that are provisioned for Lync Online there
And IF the user is not listed, make sure that he/she has a license, and if he/she still does not show up (and in the case that there has been an on-premises Lync) make sure that none of the msRTC… attributes has any values on them. They should all be cleared for the Lync Online provisioning to work.
And if you have more than five or so users that needs this clean up, there is a tool called admodify at http://admodify.codeplex.com/ that is super simple to use that can clean this up for you in minutes.
This tool is also perfect to use for any bulk changes that needs to be done over many different OU´s for example.