Demystifying Hybrid Free/Busy: what are the moving parts?

Hello Mirela , 

I have a scenario and couple questions 

Here is the following configuration of the two organizations:

Organization 1:

Exchange 2013 CU23

Organization 2:

Exchange 2016 CU19

Both organizations have an internal connection between them and a Full Trust Relationship.

What is the best way to set up the organizations so they can share their free\busy ? 

The two separate organizations they will both be moving to O365 in the same tenant later this year. If we set them up now to use Microsoft Federated Gateway to share free\busy information what happens when these users start to move to O365, will the on-premise users lose the ability to see free\busy for those users from the other organization that have moved to O365?