So the mufflers need to come off the bike to do this?
No they don't-------------
I have done this many times for many friends of mine both on stock baffles and fullsac baffles. Read on:
I have a 1/4" length of rod with a small hook at the end, actually both ends , but one is only about 1/2" , the other end is about 1.5".
On stock mufflers I grind out the welds, take a blunt end punch(big) and hammer and knock the core in about 1/16-1/8 " , then gently grind out the rest of the slag, buff with emory cloth all the way around the muffler end til its smooth as a babys butt, then insert the small hook end of the rod into the core till it hooks in one of the holes and slowly tap the other end of the rod with a hammer til the big end of the core is out of the muffler and then pull it on out. No problem. The newer stock baffles have an "inner core " with 3/8 holes in them for about 8" or so up in the core so its easy to drop the hook end of the rod into one of them.
With fullsac baffles the rod will still hook into one of the ridges on the inside of the core so you can repeat as above WITHOUT REMOVING THE MUFFLERS. Don't get in a hurry and don't try to beat them out in 3 or 4 blows, just keep tapping on the long end of rod and the big end of the baffle will come right on out.
Again , I have done this several times on my own bike both with the stock baffles the first time and with Steves many times since and have installed several sets for friends of mine.
You can also slip a piece of pipe over the 1/4" rod before you make the second bend in it and you have a homemade jerk hammer.