If an opponent uses En Garde on a unit that's been targeted by Blood Money, does Blood Money get to choose a new unit to target?

No, Blood Money does not get to choose a new target.

According to Rule 355.10 and the established mechanics for targeting in Riftbound, a spell’s target is locked in the moment the spell is finalized on the chain. If a target becomes invalid (for example, if a unit's might increases to 3 or more after Blood Money is played, meaning it no longer satisfies the "2 Might or less" requirement), the spell cannot simply switch to a new target.

Furthermore, per the Riftbound FAQ (as seen in the ruling for Hidden Blade and Shakedown), if the original target of a spell is no longer legal or is removed, the spell resolves to no effect; it does not allow the controller to pick a different target.

In your scenario, if Blood Money is played, targeting a unit with 2 Might or less, and then En Garde is played in response to give that unit +1 Might (bringing it to 3 or more):

1. The Blood Money is locked onto that specific unit.
2. When Blood Money resolves, it checks the target again. Because the target no longer meets the "2 Might or less" requirement, Blood Money will resolve with no effect.

This is my interpretation based on standard targeting rules (Rule 355.10) and similar FAQ rulings for targeted spells. No official FAQ exists specifically for this Blood Money/En Garde interaction.