Would you consider a returned payment fee a significant term under 1026.40 that cannot be increased?

I would. See the commentary:

Paragraph 40(f)(3)(v)

1. Insignificant changes. A creditor is permitted to make insignificant changes after a plan is opened. This rule accommodates operational and similar problems, such as changing the address of the creditor for purposes of sending payments. It does not permit a creditor to change a term such as a fee charged for late payments.