Change in condo assessment fees raises back assessment questions

My condo association has an owner who has been overpaying his condo assessment fees for 16 years. There was an unofficial change to the covenants, conditions, and restrictions (CC&R) stating that everyone would pay the same fee regardless of ownership since the difference was/is negligible for all units except the only one bedroom.

Does my condo association owe the one bedroom unit for all back assessments? Is the unofficial amendment (not filed with the clerk), voted on by the board long ago valid? Is there a statute of limitations for back assessment? He hasn't paid in 9 months and this issue needs to be resolved.