I don't think there are too many differences. hal Miller pretty much looks like matt Robinson without a mustache, and Roscoe orman looks like matt Robinson without hair. For the third gordon, they could have used the replacement as an opportunity to talk about being bald.
I think I have read that it was explained that oscar went on vacation to the swamp and it was so muddy that his fur changed overnight. I don't think the secodn Big Bird looks too different from the original, so it probably didn't need to be mentioned. I think his personality was just changed, but he had the same basic look, and isn't too different from the various ways that ernie, bert, cookie monster, grover (not counting the green grover), herry or elmos looks have changed.