if i play a heron to star spring. and i have a heron there already. do both trigger or does the effect occur after its moved?

Yes — both Herons trigger, and Star Spring triggers as well.

When you play the second Astral Heron (as your first card that turn), each Heron's "When you play your first card each turn, if I'm at a battlefield" checks at that moment — the new Heron has already entered Star Spring, so both are at battlefields and both conditions are met. Star Spring's "first time a non-token unit is played here" also fires, letting you move the other Heron (the one already there) to base.

You control all three triggers, so you choose the order they go on the chain (Rule 383.3.d), and they resolve LIFO. Moving the old Heron away via Star Spring doesn't cancel its already-triggered discount — once on the chain, a trigger resolves independently of its source, and "your next card costs [2][A][A] less" doesn't require the Heron to remain on board. So you get two pending discounts on your next card and the option to bounce the old Heron, in whatever order you set the chain.