When changing the AOS service account you will often find you have permission problems with TempDB. There is a stored procedure on the ‘master’ database called CREATETEMPDBPERMISSIONS_<AOS INSTANCE>. There will be a stored procedure for each instance of AX installed.
Within the stored procedure there are seven references to the old service account. You need to changed them to the new service account, and run the stored procedure before starting the AOS with the new service account.
A special thanks to Jim Willsher for the information.