For Blood Money, do I choose the unit on cast or during resolution? Let's say I target a lonely Poro first, then the opponent uses Discipline on it, can I change target?

You must choose the target for Blood Money when you play it (on cast/finalization), not during resolution.

According to Rule 355.6 and Rule 355.8, you must choose valid targets for a spell or ability when finalizing it onto the chain. Once a target is declared and the spell is on the chain, that target is locked in.

Regarding your scenario:

1. Targeting: You play Blood Money and target Lonely Poro. You must confirm it is a legal target (i.e., it has 2 Might or less) at the moment you finalize the spell.
2. Reaction: Your opponent plays Discipline in response, giving Lonely Poro +2 Might.
3. Result: You cannot change the target. Blood Money remains locked onto the Lonely Poro.
4. Resolution: When Blood Money resolves, it checks its targeting requirements again. Because the Lonely Poro now has 4 Might, it no longer meets the requirement of "2 Might or less." Consequently, Blood Money will resolve with no effect (it fails to kill the unit and does not play any Gold gear tokens).

This is based on standard targeting rules (Rule 355.10) and the established mechanics for targeted spells in Riftbound, which dictate that targets are locked upon finalization and spells resolve with no effect if the target becomes invalid before resolution (as detailed in similar FAQ rulings, such as FAQ 10047 and FAQ 6814).