Capital One David's Bridal Credit Card Not honoring payment arrangement Internet

I made an arrangement with Capital One to pay off my account making 3 payments of $280.  I made the first 2 payments, and called to reschedule the final payment, asking specifically if changing the date of the final payment would affect the payment arrangement.  I was told, "No, ma'am; it will not."  

I asked specifically if the account would be marked as paid in full on the new arranged date, and was told that yes, it would.  

I called back to ask why my newest bill was not reflecting the arrangement and still showing a balance of almost $1,000.  I was told that since I changed the final payment date, I voided my agreement with them and the account was back to the previous standing, which was MORE than what it was before I paid the 2 payments of $280! 

I asked to speak with a supervisor and he told me the same thing - that my arrangement was voided.  I was given NO notification that it was voided, was not called about the account.  No contact was made with me about it, AND I was told that my arrangement would not be affected. 

Now, I'm being told that even though I've been making payments of $280/month, almost triple the minimum payment, that my account will still go to collections and be possibly written off.  

How is this legal????