Help! Making mittens, need to reverse the thumb position

Hello again! I am making the easist mitten pattern, except that I can’t figure out how to reverse the position of the thumb. Truthfully, I can’t even understand why I have to! The first mitten seems to fit on either hand, so why should it matter? Anyway, there are 24 sts on three needles. For the thumb opening I “k 7 sts, slipped the last 5 sts worked to a holder, completed the round then casted on 5 sts over the 5 on the holder.” How do I reverse that?

The reverse would be to knit to the last 12 stitches, slip 5 stitches onto a holder and then knit the last 7 stitches. Then k7, cast on your 5 over the holder and complete the round.

As for the why, I’m not that glove or mitten saavy so I really can’t say.