Assigning a license to a user is pretty straight forward in Office 365, however if you ever need to change a license be careful of the following.
When you view a users licenses you will see something like the above. Now let’s say that you want to reassign an Exchange license so you uncheck the option like so:
Now, if you press the save button without assigning another Exchange license you’ll see the following:
If you proceed all the users Lync contacts will be removed but what about the mailbox?
From what I have been able to see is that if you do indeed make a mistake and fail to assign an Exchange license you can go back in and assign an Exchange mailbox to that user and all their mail will still be there. Now I can’t say for sure how long that will happen for but judging be the above message you would think 30 days, however I don’t I’d be willing to leave it that long.
So the moral here is, if you are adjusting Office 365 licenses between plan make sure you have assigned a license for everything that user required BEFORE proceeding and saving changes. If you don’t you run this risk of losing their data, however from the testing I have done it seems that if you correct your mistake quickly enough you should be ok.
Best advice is always to check before proceeding.