Disappointing Cover changes

I am not very Happy with one plan. I have been with them for 4 years and mainly claim for routine care. When I took out the policy they covered dental scaling and cleaning now they say do not cover it and my vet requires it. How are insurance companies allowed to just change the cover of the policy without telling you when your policy schedule says otherwise on the onset of cover. Yet the price of the cover doesn't decrease but only increases. I am very upset with oneplan and actually considering leaving as this is such a waste of money now.