It's of course pure speculation since it sure ain't gonna happen. But:
1. It may go nowhere at all since the wings may not have been designed to accommodate the heavier weight of the PWs.
2. At least the outer engines will for ground clearance have to be raised somewhat higher on the wing. That will give a drag penalty which reduces the range.
3. The heavier weight of the engines will reduce combined fuel and payload weight. Due to altered weight distribution it may affect max fuel load downwards. Maybe not.
Apart from that, present day CFM56-5Bs are a lot more efficient than the 25 years old CFM56-2 on DC-8-73. And we are told that the new PW
is some 16% better than the -5B. So ferry range will surely be increased substantially. (As well as 20+ years old -5As on old 320s are no match for present day -5Bs - the -2 and the -5A are in fact a lot more similar than -5A and -5B).
But with a meaningful payload, due to increased OEW, max fuel load will be reduced substantially. Whether that will mean increased, unchanged or decreased range will demand a qualified analysis. But sure it will be more fuel efficient.
At the end of the day, re-engining an airliner is a lot more than just bolting on another blower.
Therefore the 320neo, which is going to take to the sky with the new PWs, will be modified substantially. Wing structure reinforced, and a whole lot of other changes to allow an increased MTOW as compensation for the increased OEW. And it is highly unlikely that any non-neo 320 will ever in the future be upgraded to 320neo. If viable, that would make a lot more sense than doing the same to the DC-8-73.