I've changed the fluid and filters myself many times on both the 545RFE and 45RFE, never had a problem with bolts even seeming stuck let alone breaking any. It is a very messy job, there is no drain plug so you just loosen the pan and let it gush everywhere. I got a very big catch pan just for this. It's more likely someone will strip out the threads reinstalling the pan if not careful. the 545RFE (transmission on 5.7L Commanders) pan bolts are speced at 105 in/lbs torque (8.75 Ft/lbs) so just barely tight. Does this mechanic of yours have some previous knowledge of you car that they don't want to come to light?