I did the leg in ribbed and switched to stocking net for a half inch. The leg was done in the round.
I then did the heel in flat but continued the stocking net pattern. The heel stocking net sitch immediately became larger at the transition for the same number of stitches and, consequently, flared out because each stitch was larger when done flat than when done in the round.
For something like this you can see if your purls are looser or tighter, then do the flat section with a smaller or larger needle so the gauge will match the in the round part. If you decide to take it out and do it like this, you can check the gauge to see which size needle you'd need to use.