Ethics and the right to a legal remedy are two different things. Unfortunately, until you sign papers, you have no legal agreement. However, you may wish to take all your paperwork to a local real estate attorney to review for any loopholes. Hope this perspective helps!
I would definitely take your paperwork and consult a local real estate attorney. You don't have an agreement until the papers are signed, but there are consumer protection laws that guard against a lender doing a bait and switch at the last moment. Also, such an action may not give rise to a legal remedy but may give you good standing or negotiating a favorable resolution of whatever is currently happening.
The hardest thing to do is to walk away from a deal taht you want which has suddenly turned bad. What they did was change a term at the last second that had a large impact on you, your best course of action was to walk out, they would have either come back or not, but you would not have been stuck with a bad deal.
I am licensed in New Mexico and Pennsylvania, and therefore any discussion of issues related to other states must considered within that context. In addition, my comments are not intended to create a legal representation but merely to respond to the limited facts presented by the question. Any opinion herein is not meant as a precise statement of legal rights or as a recommendation of any particular course of action. A more complete legal review can be obtained through local counsel.