If I play Alpha Strike and choose which units I deal damage to, then if my opponent reacts with Stupefy, do I deal less damage or do I again choose the damage to units?

To answer your question: You do not choose the damage distribution again.

Here is the breakdown of how this interaction works based on the rules of Riftbound:

1. Finalization (When you play Alpha Strike)

When you play Alpha Strike, you must finalize it on the chain. This includes:

• Choosing the friendly unit that will deal the damage.
• Choosing the enemy units that will receive the split damage.

According to Rule 355.14.b, targets are chosen when the spell is finalized on the chain. According to Rule 355.15, these choices cannot be changed after this step.

2. The Reaction (Stupefy)

When your opponent plays Stupefy in response, it is placed on top of the chain. Because Stupefy is a [Reaction], it resolves before Alpha Strike.

• Stupefy resolves, giving your chosen friendly unit -1 Might for the turn.

3. Resolution (Alpha Strike)

When Alpha Strike finally resolves, it uses the current state of the game:

• Might: The friendly unit now has 1 less Might than it did when you played the spell.
• Damage Distribution: You do not get to re-choose your targets or re-distribute the damage. You are locked into the targets you chose during the finalization step.
• Splitting: Because the friendly unit now has less Might, the total amount of damage it deals is reduced. You must still distribute this reduced total among the targets you originally chose.

Summary

You do not get to choose new targets. Because your unit's Might was reduced by Stupefy, the total amount of damage available to be split is lower than it was when you first played the spell. You must distribute this new, lower total among the units you already selected.

This is my interpretation based on the rules for targeting (Rule 355.14) and the resolution of effects. No official FAQ exists for this specific interaction.