I would have fought it tooth and nail if I was you. If it says in the lease that you can have a dog then what can they do about it? Evict you? Don't leave. If they send the cops, show them the lease. You did nothing wrong.
I would have fought it if I thought the dog would have actually worked out with us, but she kept stealing my daughter's food and snarling at her, so she would have had to go back anyway. I don't live there anymore and I now have 2 wonderful dogs, so it all worked out well in the end.

Other tenants that lived in the townhouses did fight it and won. They got to get dogs, or keep the dogs they had, but you can bet that there was a separate lease banning dogs when their current leases were up. That's what sucks about renting and owning pets. The landlords can change their minds any time they want. There's not really much you can do about it unless what they're doing is completely illegal.