I have never mixed them but look at it like this, you are flying down a hill and low and behold, your seals give way and you can not stop. When it comes to such things as brakes I want to be compleatly sure they will not fail when I need them most.
Personaly I would drain the system and refill it. Aslo, if it does destry your seals you will not only have to replace the fluid but also the seals. Not worth the expense in my opinion.