changevar with y=x^2

Please can someone explain this:

(%i1) changevar('integrate(x^3,x), y-x^2, y, x);
                                     /
                                     [
                                     I y dy
                                     ]
                                     /
(%o1)                               ------
                                       2
but:

(%i2) changevar('integrate(x^2,x), y-x^2, y, x);
                                   /
                                   [
                                   I sqrt(y) dy
                                   ]
                                   /
(%o2)                           - ------------
                                        2

I don't understand the minus sign.
Thanks