Is it feasible to attach an E3 license to a AD synchronized user prior to mailbox migration?

Is it feasible to attach an Office 365 license with Exchange selected to a synchronized user prior to migrating his On Prem mailbox?  Will doing so create a second, duplicate mailbox?
Anthony K O365Messaging ConsultingAsked:
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it will not

The problem is that, MS is saying that at O365 end you must have "Mail enabled user" only to initiate mailbox migration, otherwise you may face errors while migrating actual mailbox
I see few admins assigns license in between before completing migration
However The standard practise is to wait until you finish your migration and then assign license

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Anthony K O365Messaging ConsultingAuthor Commented:
This was very helpful. Thank you!
Anthony K O365Messaging ConsultingAuthor Commented:
it appears that when a user is assigned a license prior to migration and the email service is enabled, a mailbox was created in the Cloud, while the On Prem mailbox was still being used. Not sure why
actually assigning license prior to mailbox migration completion creates mail flow issues
For Ex:
user1 is onpremise mailbox and you have associated mail user in cloud
now you started mailbox migration, now what happens, in backend Exchange online must declare that users as mailbox user so that it starts accepting emails from onpremise mailbox
However they know that if its declared as full mailbox, it will create mail flow issues, during migration if other O365 mailbox user send email to user1, the mail should go to user1 mailbox with onpremise exchange, if within cloud, exchange online found user1 mailbox, mail will never go down and user1 onpremise mailbox never notified about email. By *NOT* assigning license, exchange online still treat user1 as mail enabled user and hence hybrid setup send email to onpremise user
If you do assign license, Exchange might convert user into full mailbox and it most probably will create mail flow issues
Once your migration is completed, onpremise mailbox gets converted into mail user and cloud mailbox is declared as complete mailbox

This is as per my understanding and don't have any proof documentation
Probably any Exchange MVP / Microsoft core Exchange team member can bring fact in front of us

Anthony K O365Messaging ConsultingAuthor Commented:
Excellent. That makes sense!!
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