I browse through Nonte's
Pistolsmithing book frequently, and he had this to say about caliber conversions on the 8mm Nambu...(scan attached)...
No mention about boltface or extractor modifications...So I stripped my bolt down [removed firing pin], stuck a couple Fiocchi 7.65 Luger cartridges in the mag, and cycled it a couple times...Fed & extracted OK...
Further examination convinced me that if the extractor was removed, I could measure headspace from the ejection port...
I'm not contemplating converting to 7.65mm or 9mm in the near future, but barrel replacement looks do-able...For caliber change purposes...