That's bl**dy ridiculous!! It might be worthwhile contacting the people that have sent your new form reminding them that you have only just finished the appeal process, are they SURE you need reassessing already (and if they hesitate then ask to speak to a supervisor)? It may be an oversight/admin cockup. If they insist you have to be reassessed then maybe complain to someone like your local MP. If they say no you don't need to be reassessed make sure you get something in writing. I don't know the position re the back payments, someone else on here may know, but it doesn't sound right or fair. But then since when was the system fair...?
So sorry you're having to go through this, I hope you're able to get some support to get a decent outcome.