My personal theory is that the actor Sen Yung had a bond with Toler and only wanted to be "Jimmy" for Toler. When Toler passed he retired the Jimmy character and became Tommy perhaps retaining #2 status as a compromise.
The Chan films were very popular and even though Monogram was a poverty studio they still employed good writers. I can't believe it would have been an overlooked accident to change character names without reason. After all, Yung was "Jimmy" in the last batch of Toler films. There would have been a whole slew of people who would have noticed the change before cameras started rolling.
Yours makes sense as a possible answer. I agree that the name change was certainly not an "accident" as Victor Sen Yung probably asked the big question regarding this when he first read the script for what would in the end be titled "The Chinese Ring" (working titles: "The Mandarin's Secret" and "The Red Hornet"), the first Roland Winters Chan picture. In the end, calling Jimmy "Tommy" was probably an executive decision.
