[halfthishalfthat]

To elabourate:

変 - character signifies change, going through a change... simply the change. something was one thing, now it is different.

Perhaps you would say something like...
人生を変える - change life (as in "make a change to human life")

換 - character signifies change from current to something else, exchanging one thing for another, something in place of another.

Perhaps you would say something like...
電車を乗り換える - change trains (as in "ride a different train instead of the current one")