Yes I know I should have it written down. My situation is awkward, I joined a midsize company that has few partners. My agreement with the (verbal) was put in X years, wanted a N% stake in the company fully vested at the end of to 5 years.
Now I need to get this in writing, a new twist is the agreement wants to changed X + 5 years, for N% stake. I have a room to negotiate obviously. But If I am locked up for X + 5 years I am looking for voting rights and at least board level representation. My intention is the extra 5 years I would like to make sure that I get to vote on any strategic directions.
You are on the right track for wanting to write things down and get the deal on paper.
I would recommend reading this answer about splitting up equity. I know that you are down the road a bit from this but Joel's tenant about fairness and perceived fairness are great going in positions for your negotiations.
I would hold firm on the original deal. A deal is a deal -- written or verbal. Your fellow founders need to realize that.