Lifecycle Services is a tool set that Microsoft Services provides that will be essential to your upgrade and to your life after the upgrade. As you are going through the business process analysis, I'd encourage you to check out Lifecycle Services and look at what they have provided in their business process modeler. Lifecycle Services is a nice tool set for you to organize your business processes.
Note: Prepare model metadata
During data upgrade, one goal is to maintain element IDs between the existing AX 2012 environment and the upgraded finance and operations environment. To accomplish this goal, you must bring a copy of the element IDs from the AX 2012 environment into the finance and operations environment. AX 2012 stores element IDs in a table that is named ModelElement. This table is in the model database, which is a separate database from the AX 2012 business data database. During an upgrade to finance and operations, you must copy the AX
2012 database to Microsoft Azure. This process can be time consuming.
To avoid copying the whole model database to Azure SQL Database, use the following procedure to replicate the ModelElement table in the business data database. Later, during data upgrade runs, the database synchronization process will retrieve the required information from this replicated table and make sure that element IDs are maintained in the upgraded finance and operations environment.
* In the finance and operations data upgrade checklist, click Prepare model metadata.
* When you're prompted, click Yes.
* Wait for the copy process to be completed.
If the process is successful, the task is marked as completed.
Reference:
https://stoneridgesoftware.com/the-four-steps-to-upgrade-from-dynamics-ax-to-dynamics-365-finance-and-opera
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/migration-upgrade/prepare-data-upgrade
https://docs.microsoft.com/en-us/dynamicsax-2012/appuser-itpro/microsoft-dynamics-ax-intelligent-data-manag