Call the 800 number they give you where it says you need to register by phone. There is an automated system that will take you through entering a new number after you enter some basic info. It's pretty simple.
"Hundreds of dollars" is far greater than "almost the cost of a new Windows 7 copy". The simple answer here is "No" and you should be forwarded to the activation "operators". So long as you're starting with a valid copy of Windows 7 and have a valid product key, at no time should anyone from Microsoft be asking you for money.