You can knit it all and then turn it inside out; as long as you can translate any decrease stitches required for the hat, to be worked on the knit side. You can’t easily just turn it inside out to work stitches, and then back the other way, if you know what I mean. So if you start working it inside out, you have to finish working it inside out.
There’s a trick for working swatches for projects done in the round. If you’ll be purling the whole time, be sure to do the swatch in purling. Or in knitting if you’ll be knitting. Since making a small circular swatch requires a bigger swatch for easy measuring; the trick is to make a flat swatch, simulating knitting in the round. You do it on double pointed or circular needles. You knit a row, then slide the work to the other end, and knit the next row; repeat. You need to leave a long strand of yarn in the back so you don’t end up with a fat i-cord. You’ll have a flat swatch, with big strands of yarn draped across the back. This simulates knitting in the round, because you get stockinette stitch by all knitting. (Or all purling if you want to work that way.) Hope that makes sense.