Can a Fund change withdrawal liability calculation rules? Under what circumstances?

[ERISA-Bubs]

My client is contemplating leaving a multiemployer Fund.  We have two withdrawal liability estimates, one from a few years ago and one very recent.  The recent one is almost triple the one from a few years ago.

We have been told that the Fund changed their withdrawal liability calculation rules, removing a cap on how they calculate unfunded vested benefits.  This rule change caused the huge increase.

This seems suspicious to me that my client could have left the Fund a few years ago for a fraction of the cost based on unilateral action by the Fund.  Is the Fund allowed to make this change?  Should the Fund have provided advance notice before making this change?  Are there any other defenses my client has against this huge increase in withdrawal liability based simply on the Fund changing how the liability is calculated?