Italy 1918-20 20 Centesimi KM58 overstruck on KM28 - why?

(updated to add links to the pieces mentioned)

From 1914 - 1922, Italy issued a Nickel 20 Centesimi coin (lady holding wheat on one side and a flying Liberty on the other) - except for 1915-1918 (and only a comparative few in 1919 compared to the other mintages).

During and just after WWI, Nickel was in short supply and used for the war effort.  So in 1918, they took roughly 77 million of the old KM28 20 Centesimi (crown and wreath on one side, country and value on the other), and restamped them with a different design (Savoia shield and country on one side, value on the other) to become KM 58.

Why?

They didn't stamp them to revalue them as worth more or less than previously.  They didn't overstamp them to remove an old, despised ruler's image - the ruler at the time the original coins were issued (Umberto I) was the present ruler's father, and even then, the older coin doesn't have his name or image on it, only a crown indicating the Kingdom itself.  With everything in short supply etc during / after the Great War, why expend the effort to restamp coins with essentially the same information they already had on them (country and value)?  Why not just throw them back into circulation as they were?

The answer is likely in your own link.

KM#28 was demonetized already 13 June 1909 and withdrawn from circulation until 1914.

Thus, the government had a significant number of withdrawn pieces, but there were almost 11.4 million KM#28 unaccounted for hidden in drawers, pockets, ore simply hoarded etc. That's the equivalent of over 4 million Euro in today's money, adjusted for inflation.

Now if you just put them back into circulation, you need to make a law that re-validates them and also all the others that are still out there too, effectively giving cash money to the little guy and missing out on value of newly issued coins. No government would ever do that…

On the other hand, if you make new coins, you need to buy metal / and make rounds. When everything is scarce, why not take the perfectly fitting coin shaped rounds in your desired alloy you already have in storage?

The KM#28 pieces were also easy to fake, apparently. 

Or perhaps someone in charge wanted to gie a job to the mint so it would not sit idle, maybe in return for something else…

Thanks!  I wonder then, why they didn't go with the KM44 design?  It would have been consistent with the coins already in circulation, just a different alloy that many people wouldn't even notice.  Perhaps it didn't work for some technical reason?