Unless one of the location is a franchise (even then I think it will still be allowed, subject to the same caveat stated next), they won't forbid you from changing return location.

BUT --

Most likely, your rental rate will be recalculated and you can end up with much higher rate than you were first quoted, though since it was already a one-way reservaion, there is a good chance that you will end up with roughly the same rate. Call Hertz and they will give you a pretty accurate estimate on how much the rate will change if you decide to change your return location and/or time.