Property management firm shaking things up at WA association

I have lived a condo of 12 units for 7 years. The top 6 units have always paid $15 more each month because they have one more bedroom, and a two stall garage.

Our new management firm is telling us that all 12 MUST pay  the same monthly association fee. I don't believe they should be doing that. 

Also, the management company is not listening to our board of directors and are "taking over"? How do I stop them?