Don't change the way you treat that person in the past. If you know that person has a crush on you, then it be if you have no feelings towards her/him. S/he will be your best friend no matter what. It doesn't mean that if you find out s/he's got feelings towards you, your relationship will change. Just stay the same.
:Never wound those you can't kill; same as never be too sweet to those you can't love.