Assuming the original ticket was round-trip (and not two one-way fares that happened to be on the same itinerary), then you're going to have problems trying to do what you want to do. If you want to use it for the return flight between the same city pairs as before, then you can do so with the change fee waived per the special dispensation you say you were granted. If you wanted to use it for another city pair, then it's going to reprice it to whatever it would have been had you originally booked it that way, again waiving the change fee.

But if you want to apply the "residual value" to a completely unrelated round-trip, my understanding is that this would be treated as an abandonment of the original return segment(s). In that case, you're asking for a refund of the unflown original segments to be applied to a new round trip. In that case, they'll reprice the original ticket just for the segments actually flown, as if it was ticketed that way originally, and if that's less than what you paid, refund the difference (or make it available for the new flights). But, assuming that your original flights were a round-trip fare, you'll probably find that the one-way fare for the segments flown is MORE than what you paid for the entire original round-trip, meaning that there is no residual value to apply.

The way I read it, the special dispensation to waive the change fee is just that - it waives the change fee, but does not otherwise alter the fundamentals of the underlying fares.