Keep your equity, you don't give back vested equity for nothing(trade it by all means for something you think worthwhile). As was already said you could sue if

Keep your equity, you don't give back vested equity for nothing(trade it by all means for something you think worthwhile). As was already said you could sue if they create another company. Finally go talk to a good lawyer(pay, lots if you have to). That is probably the most useful advice you'll get on here. Your co-founders are not your friends now and will screw you over if they think they can get away with it. Maybe you could get a lawyer to write a scary letter on what would happen if they startup another company to convince them otherwise.