A monster that is Flip Summoned changes to Attack Position. A monster that is flipped face-up by an attack or a card like Swords of Revealing Light is not consider summoned at all and remains in its Battle Position.

Not all Flip Summons are a "switch" to Attack Position. You can have a face-down Attack Position monster due to the effect of Darkness Approaches. This monster would remain in Attack Position when Flip Summoned.

Another situation where it might appear like there is no change is with Level Limit Area B, because a Level 4 or higher monster would switch to Defense Position immediately after being Flip Summoned, due to Level Limit Area B's effect.