Well, him changing his name would explain it, but I'm still not convinced that it was Koyama and he changed it to Kobayashi, instead of vice versa. His very latest release is the
Virtual-On OST rerelease from October of last year, and
Sega's page for the album gives Koyama in a comple of places, but not Kobayashi. Unfortunately, we don't have any scans of the inside, but if he did change to Kobayashi, it doesn't explain why Koyama is still being used for his latest releases.