Once made, a will may be subsequently revoked or amended. Revocation of the whole, or part, of a will is typically made consciously by the testator, either by executing a codicil or writing a replacement will. Amendments which are, perhaps, few or short in nature (where revocation is not required) are usually effected by executing a codicil.
A codicil is itself a testamentary instrument (as is a will) and is thus subject to the rigours of WA 1837, s. 9 (the witnesses of the original will and any subsequent codicil need not be the same). Any number of codicils may be executed subsequent to the execution of the will.