As pot becomes decriminalized will previously convicted low level pot dealers stay in prison?

Just curious. If what they did to get convicted is not a felony anymore once a state passes a decriminalization law will they stay in prison for the full sentence they got when it was illegal or is there some appeal they can make?

The fact that people keep asking this over and over and over is proof that pot really does kill your brain cells.

(And I say that as someone who likes pot and very much wants it to be legal.)

Unless the law legalizing it includes a blanket pardon then everybody will serve their sentences net of parole.