An issue that has often arisen in the courts in connection with joint wills or even in the case of separate wills for spouses where the will instructions provide for reciprocal benefits, is whether or not the execution of such a document by the spouses carries with it an implied agreement not to alter the will without the consent of the other spouse?
Such a finding would certainly affect the true intentions of many testators.

In answering this question, one of the most basic principles regarding the law of wills is that a will is revocable, provided that the testator has the requisite mental capacity. How, then, is it possible to suggest that a person might be restricted from making changes to his or her will without the consent of another person?

Without reviewing the case-law in detail, suffice it to suggest that, even in the case of a joint will executed by a husband and wife, the mere existence of such a document will not, in and of itself, imply the existence of an agreement not to revoke the will without the consent of the other.
There must be more compelling evidence than the mere existence of a jointly-executed document in order to prove that such agreement exists. The agreement must be proved either from the will itself or from outside evidence.

The ratio of the story: if you want to ensure that your spouse does not change his/her will after your death, visit your lawyer and ensure that you have both executed satisfactory documentation indicating such an intention.