It depends on the way you are looking to migrate the users, -If the AD migration dosen`t involve Okta then after the users are migrated to the new AD all you will need to do is connect that AD to Okta (Go through the "Add Directory" option under Directory>Directory integration ) and ensure that the users are connected to the new AD (after the setup is done run an import from the new AD, confirm any and all exact matches) . Ensure that the users become mastered by the new AD and after that you can deactivate the old AD in Okta. You can do that by changing the priorities under Directory>Profile Master/
-If migration will be done through Okta then you might have to push the users to the new AD before anything else. You can achive that through Directory push. First you need to conect the new AD to Okta through the Directory Add option as detailed above. You will need assign all users of the old AD to an Okta group using group rules after that. Once the users are populated in the group you can push the group to a specific OU in the new AD (access the group in okta and go through the "Manage Directories" steps . After that just take the steps in the first option to ensure the new AD is the profile master and deactivate the old one.
For assistance with these steps, please reach out to the support team.