1) By moving the return upstairs it places the return in the hotter, stagnant zone and continually removes the hottest air as opposed to removing and re-cooling the cooler downstairs air.
2) In the “before” the cooler upstairs supply air would be “pushed” downstairs by the downstairs return. In the “after” the upstairs return depressurized the upstairs and prevented the cold air from falling downstairs. The upstairs then retained its cold supply air.
3) With the general air pattern and building pressures forcing an upward draft up the stairwell, this reduces exfiltration type pressures on the downstairs envelope. Reducing upstairs/downstairs temp gradients further reduced infiltration.
4) Leaks may be present in the extended return ducting. This would pressurized the house and reduce infiltration.
5) Moving warmer air through the handler is more efficient.
6) In the ‘after” all structural heat gain that did not get mixed would ascend upstairs and be removed by the return or be mixed with the retained supply air. In the “before” the upstairs supply air was lost to the downstairs and could not offset that amount of heat accumulation.