I’ve went through this and worse before.
If the website is not making much money write your own letter to them and get it authenticated by a notary public with a stamp to show that they have seen your id. On this letter state that you request the domain name to be transferred back to where it used to be and that you did not authorize this initial change. Also state that you are seeking legal opinion and that you will press charges if this does not happen.
The main problem is that the registrar might take the safe route and hold the domain name as is until they get a court order to be transferred back which can take weeks or months.
If the website is making money initiate the process to get a court order to get it changed back. Contact a lawyer asap.
The third route is pretend you know all about the change and then send another fax to the new registrar to get it changed to the old one, pretending that everything is good. That way they wont suspect that the domain name was stolen and they wont place it on hold.
Either way its a shit situation.
As for what you should bring to your lawyer a print off of you initially buying the domain name and that its registered to you and how long you registered it for that way he has something to show the judge.