transmission fluid looks like water '02 trailblazer
 

Checked the transmission fluid in my 2002 trailblazer (4.2 inline 6), after it felt like it was starting to slip, fluid looked clear like water. Obviously fluid and filter need changed, but my real question is will the transmission be ok after afterwards? I was told (by a self proclaimed mechanic) that once I change the fluid and filter the whole tranny will go out...