You cannot change the terms or conditions of the document, or add a signature (to show agreement/consent to the document),or change/revise a signature. However, you can fill in missing information, like an address or contact information, which does not affect the terms of the document (and which, after all, could simply be placed on a post-it note or extra sheet stapled to the document).So filling in the information: legal. Tracing over a signature, which can be taken as altering it--not legal.