The guidance sets out when you can switch:
This page tells you when people can switch into the Tier 1 (Entrepreneur) route.
Switching into the Tier 1 (Entrepreneur) route is only allowed for applicants who
have, or were last granted, leave as a:
• Tier 1 (Graduate entrepreneur) migrant
Given that you now have a start-up visa, you don't qualify
My comments are in no way meant to be advisory. I have no professional knowledge of immigration. These are based on my own experience, convictions and personal interpretation of publicly available information.