Synchronize AD to an existing Office 365 Tenant

Mwkirkfrom what you have said they have had Exchange onprem in the past, first thing I would do is Active Directory schema https://docs.microsoft.com/en-us/exchange/plan-and-deploy/prepare-ad-and-domains?view=exchserver-2019 to make sure they are current.

You then want to export the current users from M365 and add the user details to the Active Directory users (name, display name, department, proxy address etc) once you run a sync the account in M365 will be matched with the onprem account and so if a user does not have for example a department defined in onprem but does in the cloud the cloud user will get updated with no department as onprem will over write.

When you install azure AD connect match based on UPN and the accounts should match, don't forget the users cloud password will be changed to the users onprem password and I would sync a single OU at a time and drip feed the users into this OU so you only test with 1 user at a time.

One thing to point out is that while you can edit the users onprem via AD users and computers you are now going to be in an unsupported model with syncing users but having no Hybrid to manage them. if the company have E1, E3 Licenses then just install an Exchange 2016 hybrid as the license is free