Oh yes, any muffler mod needs a retune for sure. Didn't say that but that was implied. Yes, will lean out the saw without a retune richer afterwards. - Paul
Although this is more of a problem with exhaust porting, if the muffler is initially severely restricted then a muffler mod may result in there not being enough range left in the H screw to enable a retune and this can end up damaging the saw.