Agroshield
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Most of the metal that has already been removed by cutting the M14 thread is the same metal that the 9/16" UNF thread wants to occupy. There is not enough material left in the hole to form a reliable thread, especially as the pitches differ.
On a practical note, it would also be very difficult to tap as the existing thread will try to influence the path the new tap takes (cross thread it, pull it out of alignment etc.).
You could try putting an M8 bolt into a 5/16 UNC or BSW nut as a visual demonstration of the issue. The diameter and pitch differences are on a similar scale to the current question.
If you wanted to go from say M13 x 1.5 to M14 x 1.5 - same pitch but different diameters, it would be possible as the pitches are the same and you can time the new thread to coincide with the old one, so the peaks and valleys align. But where the pitches differ, you need to make sure the diameters differ so that there is minimal overlap between new and old.
Someone on here could 3D model an M14 x 1.5 thread superimposed on a 9/16" UNF thread so the problem will become clear.
As the aluminium block is thick, make a top-hatted blanking plug that goes in from the bottom - loctite it in and then put the 9/16" UNF thread into it. A version of this that does not need a lathe is not to have a brim on the hat, lightly countersink the hole and peen the filler piece over so it cannot pull out.
but if you have one go ahead, retapping could be adequate as its not a shallow thread. Or helicoil if you have one.
