I am defending with lillia at battlefield A with smoke and mirrors hidden. i have a sprite at base. opponent sends Kha'zix mutating horror in. can i react with smoke and mirrors to swap lillia and sprite, getting 2 sprites in before Kha'zix gets the +2 trigger?

No — the +2 is already locked in. Kha'Zix's "if an enemy unit is alone here" clause sits in the trigger condition, so it's checked the instant Kha'Zix gains the attacker designation, while Lillia is still the only unit you control at A. You don't get priority until after the trigger is on the chain, and the condition is never re-checked on resolution (FAQ #11163).

You can still make the swap — Hidden grants Smoke and Mirrors Reaction, and the "different location" clause satisfies the hidden targeting exception (811.1.d.2.a), so you choose Lillia at A and the Sprite at base. Resolution: Sprite → A, Lillia → base, and her move trigger plays a second 3-Might Sprite at A, ending with two Sprites there facing a 6-Might Kha'Zix. But the +2 sticks no matter what joins the battlefield afterward.