On the webform, you will not only be asked to input the establishment date of the trust but also any subsequent dates of variations to the governing rules of the trust. This is important to demonstrate the continuity of the governing rules of the trust so as to avoid any taxation implications.
The most current set of governing rules will contain the provisions on whether the trustee can be changed i.e. removal and appointment clauses as well as who has the power to make the trustee changes.