my question would be, why even change the differential fluid? shouldn't have to and it's a much bigger pain in the ass to do then it's worth...but yes, idiots typically round the 4 sided drain plug off so you can't get it out any longer. It's a classic problem...the old diff plug..what I have seen is that folks take the bottom plug out, via whatever means, drain the fluid and then discover that the filler plug won't come out, the one on top, so they can't refill the damn thing...either way you/they are screwed....I wouldn't even mess with the differential fluid...if the thing goes it goes....and you replace the entire diff.....best of luck