In D365FO there has been added a lot more validations when trying to remove an account structure from a ledger. These validations makes it next to impossible to replace and existing account structure in a company that is live. Validations on uncommitted tax and source documents means that we have to delete all lines on Purchase orders that post to an account in the structure you want to replace. For a company that is live, it is not a viable solution. It should be possible to add a replace function to account structure and revalidate/update existing source documents and uncommitted tax, and other places that would stop a removal of an existing account structure. At the very least it should be possible to read from the warning/stop message what records are causing the stop.