Ok let's straighten out a few things:

-It's not "ex ipso facto," it's "ex post facto", and it's relevant to criminal punishment, not contract law.

-The Rule Against Perpetuities applies to conveyances, not contracts, and in any case has an exception for charities, such as the BSA. In addition, the Rule was repealed in Pennsylvania in 2006

-Finally, let's get back to the original point: the Contract Clause of the Constitution. Clown_nose's answer at 1:34 completely misses the point of the contract clause. Read Trustees of Dartmouth College v. Woodward, 17 U.S. 518 (1819), or even more on point, United States Trust Co. v. New Jersey, 431 U.S. 1 (1977), in which the Court held that the state of New Jersey tried to pass a law repealing a covenant the state had entered into, the state violated the Contract Clause.