I don't think there's ever been a precedent for that. It would be like if we had proven that Obama was born in Kenya, but he'd already been in office for two years.

The Constitution doesn't provide a provision for what to do if material election fraud is discovered after a winner has taken office. If Biden were proved to be complicit, then he'd need to be impeached and removed from office. Which probably wouldn't happen.

Honestly it would be a constitutional crisis the likes of which we've never seen.