Considering you are a co-founder and jumping ship instead of pivoting, I'd want my shares back too. I'd also be leery of accepting you on my team if you are so eager to bounce based on traction alone.
I'm not a lawyer, but I think your best bet is to negotiate something and get it in writing so you don't have it lingering over your head or negatively impact your new company.
I'm not a lawyer, but I think your best bet is to negotiate something and get it in writing so you don't have it lingering over your head or negatively impact your new company.