California is more forgiving than most, and became more so after the Supreme Court decided Haggerty v. Thornton. The court held that under Probate Code section 15402 a trust may be modified using the section 15401 revocation procedures unless the trust sets out a method of modification and explicitly makes it exclusive. The statutory method under section 15401(a)(2) is a writing other than a will, signed by the settlor and delivered to the trustee during the settlor's lifetime. That requirement is real. An amendment found in a desk drawer after death, never handed to the trustee, fails even where intent is obvious.
Texas takes a short statute and a hard line. Property Code section 112.051(b) allows the settlor to amend a revocable trust but forbids enlarging the trustee's duties without the trustee's express consent, so an amendment handing a corporate trustee new discretionary powers needs a countersignature to bind it. Subsection (c) requires a written amendment whenever the trust was created in writing. Texas also revokes certain transfers to a former spouse on dissolution of marriage under the provisions beginning at section 112.101.
Florida is where amendments die. Section 736.0403(2)(b) provides that the testamentary aspects of a revocable trust executed by a Florida domiciliary are invalid unless executed with the formalities required for a will, and section 732.502 requires two attesting witnesses who sign in the presence of the settlor and of each other. In Kelly v. Lindenau, 223 So. 3d 1074 (Fla. 2d DCA 2017), two witnesses watched the settlor sign an amendment but only one signed it. It failed, and reformation under section 736.0415 could not cure the defect. Confirm both witnesses actually sign before anyone leaves the room.
New York requires the amendment to be in writing, executed by the person holding the power to amend, and either acknowledged before a notary in the manner used for recording a conveyance of real property or signed before two witnesses, unless the trust provides otherwise. EPTL 7-1.17(b) adds a step people skip: if the signer is not the sole trustee, written notice goes to another trustee within a reasonable time. Skipping it hands a hostile cotrustee an argument.