Easy. The most common thing I see is to substitute the first two or three pages (however many there are), and keep the last page, which is the signature page, which everyone witnessed. Then the first few pages are rewritten. It's hard to detect unless suspected, and then an expert has to be able to show that a different paper was used,or different type, etc. Hard to believe an attorney would do so, is it possible that someone else is "in league"?