The only problem I can foresee changing these into mittens is keeping the pattern continuous as you increase/decrease. But perhaps you will have already had some experience on how to increase and decrease in pattern before you get to the mitt part.
I would recommend swatching out a few increase and decrease rows in pattern (or, rather, in patterns plural) AND I would suggest charting out beforehand how many times you will continue each pattern repeat (if you plan to do so).
Great choice of mittens. I hope your venture is a successful (and fun) one!