'Immediately after rolling your attack dice, you must change all of your crits to hits.'

Does this happen before I manipulate my attack dice with Focus, TL, etc?

How would this be impacted by Mercenary Copilot?

"When attacking at range 3, you may turn one of your hits to a crit."

It occurs before you modify dice. That means that MC can turn a die into a crit afterward. It also means that if you reroll because of a target lock, or pilot ability, or etc, any crits rolled that way stay crits.

Does this happen before I manipulate my attack dice with Focus, TL, etc?

Yes.

How would this be impacted by Mercenary Copilot?

"When attacking at range 3, you may turn one of your hits to a crit."

Kath + Mercenary Copilot + HLC was actually one of the ships included in the (unmodifiable) build for the Imperial player in the finals of the Kessel Run tournaments prior to the Wave 2 launch. Merc Copilot happens during the modify dice step, so it happens after the HLC switches crits to hits.

(Mostly ninja'ed by Audio Weasel, but posted anyway.)

What about HLC on Rexler Brath?

What about HLC on Rexler Brath?

Rexler's ability is used after you attack, so yes, you would be able to use his ability.

It occurs before you modify dice. That means that MC can turn a die into a crit afterward. It also means that if you reroll because of a target lock, or pilot ability, or etc, any crits rolled that way stay crits.

Dat Space-Weasel knows!

Yup because now crit-stuff is being done because of the coolness-stuff, and not the actual mechanical function of the gun-stuff.