Party affiliation has no effect on who you may vote for during the general election.
Registering Green or Progressive Party will send a message to the Democrat Platform Committee that the Democrat Party must earn our vote.
After the election you may keep you registration or register with any or no party.
However, you must register alternative party before the Democrat Convention to have an effect on Platform Committee decision making.
As you may know, I endorse Jill Stein of the Green Party
Code Pink Power to the People