One cannot change the Funding Method if the filing deadline has past.

Is there such a restriction on changing the assumptions?

What if Form 5500 was filed but the Sch B was not?

You can not change assumptions once the Sch. B has been filed.

Are you saying they just didn't file the Sch B with the rest of the filing?

Yes, that's what I was trying to say - Form 5500 was filed but Sch B was not (because the Sch B was not prepared).

I was also trying to generalize the quesion into two questions:
1. As above, Form 5500 was filed but Sch B was neither prepared nor filed.
2. What if both the Form 5500 and Sch B were not filed.

to see if it makes any difference.

You can change assumptions in your case.

While the IRS is fond of saying you can't change assumptions once a Schedule B has been filed, the actual rule is not quite so strict. Almost, but not quite. You are allowed to change assumptions to the extent that the new information you are processing would naturally lead to a situation where you would sort of have to make that change. Doesn't come up often, but when it does, it is good to know that it is not impossible to change assumptions.