The rule: a trust deed is a contract for a third party
South African law treats an inter vivos trust deed as a stipulatio alteri, a contract between the founder and the trustees for the benefit of third parties (the beneficiaries).
Before acceptance, the deed can be varied without the beneficiary
A beneficiary is not originally a party to the deed. Until they accept a benefit conferred on them, the founder and trustees may vary (or even cancel) the deed by agreement, within the limits of its amendment clause. The beneficiary holds only a hope (a spes) that the law does not yet protect against variation.
After acceptance, the beneficiary's consent is needed
On acceptance the beneficiary acquires a right under the deed and, in effect, becomes a party to it. From then on the deed cannot be varied to their prejudice without their consent (Crookes v Watson; Hofer v Kevitt; Potgieter v Potgieter). In Potgieter the court was explicit: whether the accepted right is enforceable, conditional or contingent makes no difference, what matters is that acceptance created a right worth protecting. And acceptance is not selective: a beneficiary who accepts a benefit takes it subject to the deed's limitations, not just its upside.