Can I change a creature from attack position to defense position or vice versa only during either of my Main Phases, and only once per turn per creature? Is that correct?
Is that correct?
Yes, it is correct.
Unless otherwise specified or restricted, you can do that what you describe.
You can't, however do so on the turn you Summon a monster, as once you place it on certain position then it has to remain that way until you can legally change it again (either by card effect or rulebook effect). Think of it as the "summoning sickness" that MTG creatures have, just that it applies to position instead of attack permission.