I honestly can not remember many places were Pi was on it's own. If you can please do mention them.
A major part of the point of what I guess you could call the 'Tau movement' is that it is not at all incompatible. That's why (and again the Tao manifesto covers this) they are suggesting using Tau and not changing the value of Pi. You would simply be introducing a new constant that everyone could recognize as 2 Pi. Books could also have the two different versions of the formulas on the same page of their text books and they should be able to function just fine.
To be honest I actually do have one problem with the idea, Tau is already heavily used in other areas (it looks to much like a T to be ignored), another less used Greek letter should be chosen instead.