Possibly a dumb newbie question regarding NPNs

[Terrence Evans]

Ok so let's say I buy a a non-performing note for 30 cents on the dollar and I know that the area is someplace relatively depressed like say Detroit.

Let's say that the balance was $30k and the property is worth half that.  What is to stop me, the buyer, from throwing out the new balance and just creating different terms more appropriate for that property?  Do you have to be registered to do so?