Once a trust has been established by a Deed, if the terms of the trust provide an amending power, then under the common law only another Deed can be used to amend the terms of the trust.
Therefore if more flexibility is required to amend some or all of the provisions of the terms of the trust, the use of an alternative instrument must be prescribed in the original Trust Deed that established the trust.
For example the original Trust Deed may prescribe that a Deed must be used to alter the provisions related to the beneficial interest of the beneficiaries under the trust, however an instrument under hand might be used to alter the administrative provisions such as what are authorised investments.
The common law requirements to amend another Deed were discussed in ING Funds Management Ltd v ANZ Nominees Ltd; ING Funds Management Ltd v Professional Associations Superannuation Ltd  NSWSC 243 at [72-74]:
72 In the case of a deed inter partes – in essence, a deed embodying the contract of its parties – there can be no variation except by another deed. The common law rule was stated by Bosanquet J in West v Blakeway  EngR 591; (1841) 2 Man & G 751; 133 ER 940 (at ER 949): a contract under seal cannot be varied by parol contract.
73 Tindal CJ (also at ER 949) referred to the maxim unumquodque ligamen dissolvitur, eodem ligamine quo et ligatur (or, as it appears in R H Kersley, “Broom’s Legal Maxims”, 10th edition (1939) at 592, nihil tam conveniens est naturali aequitati quam unumquodque dissolvi eo ligamine quo ligatum est: “nothing is so consonant to natural equity as that every contract should be dissolved by the means which rendered it binding”). The Lord Chief Justice then said: “But in the case of a covenant the whole matter is under the seal of the party; and the contact into which he has entered can be discharged only by an instrument of the same nature as that by which the contract was created.”
74 The matter was put thus by S M Phillipps and A Amos in “A Treatise on the Law of Evidence”, London, 1838, at 774: “Where, however, the parties have defined the terms by a writing under seal, (which must be taken to be made with great care and formality,) the policy of the law will not permit it to be altered by matter of a lower nature.”