How can we smoothly migrate our Exchange 2010 email domain to our parent company who already has hosted Exchange with O365?

Parent has new Hosted Exchange domain, “”. We are “”. We were advised to, and have, created new users in parent domain both AD and Exchange. That gives us access to both mailboxes in Outlook. Then have users move their created “” folders to the new parent mailbox.
Then add the “” domain to the O365 tenant and create “” aliases for all the “” mail boxes.
When ready in O365 portal, change MX record for “” to go to the O365 tenant for “”.
This way existing senders could send to the old “” and mail would flow into the “” box. The idea here was to end up with one mailbox that would receive (and send) from either domain. We currently now have both mailboxes in place but many questions arise:
How do we handle shared mailboxes and resources, conferences rooms? How can we gracefully move our old Calendar events and meetings? Do these things all need to be recreated? That is our understanding with this method.

But, what about just migrating our existing domain, “” into the O365 tenant and keeping everything as is? I know there are automated methods like "remote move" and a PS script. However, we have heard some bad horror stories about lots of errors, NDR's, Calendar events and meetings not working, etc...
Sorry to be so lengthy on this. I think that I favor the first method (new mailbox that can handle both domains). But our users are not happy and execs are not willing to have to redo all meetings and Calendar events and any other customizations. So, I need to consider the second "true migration" type method as well.
If you have a lot of experience with Hosted Exchange Migration I would love to hear your comments on this.
Thanks so much!
Who is Participating?
RoninConnect With a Mentor Commented:
The "True Migration" approach is to consolidate both environments, DirSync (AADConnect) to Office365 and move mailboxes. All other approaches will result in bunch of lingering behind, such as meetings, loss of permissions, wrong cached entries, etc.
Jason CrawfordConnect With a Mentor Transport NinjaCommented:
I started to write out some recommendations and considerations, but really this very well written support doc from the manufacturer does a better job than I ever could:

Whatever method you decide on you need to stop using the one you described in your initial post.
TMITECHSAuthor Commented:
Thank you Ronin so much for this information. It looks like AADConnect is the new version of DirSync, right?
I see that you are familiar with the possible bad consequences that can occur sometimes.
I need to learn more about this. One complication is that we are not migrating alone from scratch. If so, then I could choose all the options and methods on our own but I need to be mindful of our parent company that has already started this process and be both compatible and compliant.
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TMITECHSAuthor Commented:
Thank you Jason.
I see the link that starts the deployment process. I guess I would choose Hybrid just to take it slow.
But our parent has already established the tenant. We need to join them in the most reliable way.
I will go through the deployment information. Thanks very much for contributing.
AADConnect is the only supported method of on boarding accounts to Office365.
See the supported topologies, depends on how the parent company is configured, you might be able to do it. Otherwise, maybe create trust between domains and move the users through their organization.
TMITECHSAuthor Commented:
Thanks both. I will just wait a day or two to see if any more comments come in before  closing this.
Any updates?
Mohammad Ishtyaq KhatriCommented:
Hybrid Deployment

Checkout this URL - You can try having a multihybrid approach. But it also involves your parent company as well weather they want to configure a hybrid with their tenant.
TMITECHSAuthor Commented:
Thanks all, still in limbo due to no consensus among management.
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