I've had that kind of request. Albeit without anything close to that kind of sales figure afterwards.
It's always a passing thought as I've been making these changes, but one that occurs to me very often.
I've never really tried testing the theory though.
And just to contradict myself, there are times when I'm surprsied how minute the changes can be to bring something out the mix, or to not have something too loud.
A lot of the changes I have to make or the small, usually automation type things you do as you get close to finishing the mix.
The things that get done once a song is mostly mixed, and you just keep listening to it to finish it off. Minor volume changes to suit the part and the arrangement, etc.
I just wish I could do that when mixing the track rather than having to come back and spend a day making minor changes to every song a couple of weeks later.