The Ki100 was a newer and improved version of the KI61.
How did it get worse?
Not exactly. The Ki100 was not a newer and improved version of the Ki61. Think of it as a Ki61 with a
different engine.
What happened was that B-29 strikes had destroyed the factory that produced the engine for the Ki-61-II (which was superior to the Ki-100 in almost every way). So there were several hundred completed Ki-61 airframes sitting around with no engines.
Japan in its desperation for aircraft to defend against the American bomber strikes started looking for ways to get those aircraft into action.
Their solution was to fit a smaller, lighter, and very reliable, but less powerful radial engine which they already had in production to the existing airframes. And the Ki-100 was born.
This produced an aircraft that was very reliable, during a time when production quality and reliability was low throughout the japanese airforce. This plus the extreme manueverability (something the japanese always valued) gave birth to the history that produced what I call the "Ki-100 Myth" that you see in the online sim BBS's.
The Ki-100 was a cobbled together design that was obsolete and outclassed from its first day in service. The Ki-84 and the Ki-61-II were the best that the Japanese Airforce ever produced in WWII.