I just found out about codidact, and have spent the past almost hour reading, so apologies if I’m missing something important. I’m hoping to get involved, but a tad overwhelmed at trying to catch up!
It seems like there is consensus reputation will be different so trying important rep won’t work anyway. I agree with @cellio that giving some type of privilege for existing users, albeit not rep on the site, will help drive engagement. I’m also okay with no rep but privilege tracks or “stars” as someone else said it. But all this means we kinda have to go with a new-start paradigm.
Also, I think asking users to place a special code in their SE profile, or some other objectively verifiable mechanism would be wise. It’s best to avoid issues of people trying to steal other user’s names and privileges.
Edit: it looks like SE has an SSO option, so we can probably leverage that to tie accounts together, as is commented over here