The primary difference between parallel conversion and direct changeover conversion is that

With parallel operation, one branch, department, or division at a time is fully converted to the new system.

With parallel operation, one function of the new system at a time is placed in operation.

With direct changeover conversion, the old system is shut down and the new one takes over processing at once.

With direct changeover conversion, the old and new operating systems are both run at full capacity for a given period.