If you are overfilled that amount when the fluid is hot I would not be concerned. That is miniscule.
As to the diff change out. Don't worry too much, other then getting out an extra quart or so of fluid and cleaning the magnet on the drain plug, (my Camry diff plug is a magnet, tranny is not)it does not make much diff. The fluid does mix, and perhaps another drain and fill in 20,000 miles would make you feel better. Also, don't pull the drain plug until you are sure that you can get the fill plug off first on the diff. I measure my tranny fluid after it has drained because as many times as I have done this I always get a different amount of fluid out and I simply replace it with that amount of new fluid. Not measuring (unlike oil which is consistent as to how much is drained out) creates the potential overfill issue.
Always check the fluid hot after you have a shop do it because they rarely have the time or desire to drive the car 10 miles, heat the fluid up and check it. They go by the word "Assume" it is okay