ADDS trusted forests.domains A. OnPrem EX2013 B.Office 365 into new ADDS and New 0365 Tenant?

[Floyds_on_Greenwood]

Greetings.
We have two company's (each with their own forest and single domain) that have operated in a trusted ADDS forest configuration. Each forest contains their own respective mail system. One has on-premise Exchange 2013. The other ADDS forest has O365 and uses Azure AD Connect to sync on-premise ADDS users to o365. These mail systems are utilizing Galsync (enow) to support cross forest GAL's.
We are not (yet) using o365 SharePoint, one drive, or other 0365 services other than email. *We will later in the new named entity.
We are now going to merge these two environments (ADDS forest(s) / domain(s)) into a new named ADDS entity (forest and domain) - and new o365 tenant. This new named entity will utilize many of the o365 offerings.
I have migrated/merged trusted forests, and Exchange on-premise 2010/2013 systems together via ADMT and mailbox moves. This looks to be a bit more challenging.
Has anyone performed a similar migration/merge? Would they be willing to share how they did it?
Any insight, links, or thoughts are very much appreciated.
I found something similar in this forum on reddit -https://www.reddit.com/r/Office365/comments/93f4oq/cross_forest_office_365_migration/
Thanks in advance,

[#2]

Hello
So you are going to change UPNs , Primary SMTPs etc? right ?If you think of it, the whole process looks like a scary one , but really it is quite straightforward . First things first , AD Sync is your friend and the very tool that is going to keep things in line . Create your new Tenant , and verify the Mail Domains. You will go Hybrid so people will continue to work, and maybe in the end you may Decommission Exchange 2013 right ?

[#3]

@KonstantinosPassadis 

Yes, you are correct.  We are going to change UPNs , Primary SMTPs etc and we will decommission Exchange 2013.

Would you recommend something about the process that might help?

Thank you.

[#4]

Hello@Floyds_on_Greenwood 

Sorry for the late answer , i was quite engaged. So have you started yet ? One this i would like to point out is to carefully plan your Identities , just make clear which one is which and what aliases are on them

What about your PST files ?  Are they holding a lots of GB ? In a similar case we uploaded all Data and made the Import Jobs from 365 Admin.

[#5]

@KonstantinosPassadis
We have not begun.
PST's should be relatively small.
Thanks fro your insight. Much appreciated.

[#6]

Hello@Floyds_on_Greenwood

Well if you allow me to summarize

Verify domains in Azure or O365 , keep in mind it MAY take 24 hours to move from one Tenant to another , plan before hands and add extra waiting time just in case

Check external guest identities that they don't make duplicates with actual AD users

Add UPN Suffixes to your Domain/Forest

Install/Upgrade AD Connect , provision users on new Tenant

Add Authoritative Domains  run Hybrid Wizard and check all domains , select centralized mail transport if you keep your MX onpremise

In the meantime export all PSTs and plan for the Import procedure with AzCopy

https://docs.microsoft.com/en-us/microsoft-365/compliance/importing-pst-files-to-office-365?view=o36...

Let me know your thoughts !