Changing a hat pattern from straight needles to round

[Whisper-Ma-Phone]

Hello all. It looks like you've got a great site here and I'm happy to be a part.

I knit up a baby hat (I'm expecting) and the pattern used straight needles. I would like to try again knitting in the round instead to get rid of the bumpy seam at the back.

So, the question is, how do I change my pattern from straight to round? Would you cast on the same number of stitches, or cast on less stitches because there will no longer be a seam?

If you need pattern details, I can provide them.

Thank you!