When spouses/partners sign simultaneous wills that contain identical provisions, can the surviving spouse change his/her Will contrary to the agreement with and wishes of the earlier-to-die spouse?

The answer is "probably unless...." A will is always presumed to be modifiable and revocable. However a will may by its language evidence an irrevocable agreement between the parties involved in making the will, but the irrevocable agreement must be definite, clear and unequivocal.

Even where there is a common wish between the parties at the time of the will's execution, there must be evidence of an intent to make the will irrevocable with respect to either party. The existence of reciprocal wills is not sufficient by itself to imply an agreement between the spouses that the wills were to be irrevocable. Without any expression of or reference to an agreement not to revoke, no promise to such effect will be implied solely by reason of the reciprocal nature of the testamentary provisions.