If you want to migrate a unit into the child account and also change unit creator onto this account creator please use account/change_account command:

svc=account/change_account&params={"itemId":<long>,
                                   "resourceId":<long>}

In order to migrate a unit into the child account this account creator user should have some rights:

1. On his account:

2. On migrated unit:

If migration ended successfully then:

{}

If some errors occured:

{
    "error":<uint>
}