Can you change aircraft before a sortie?
Yes. Official walkthrough footage places aircraft, special-weapon, part and squadron selection in the briefing and sortie-preparation flow.
Confirmed explanation
The system is designed so the player can respond to target classes and mission conditions before launching. Two special-weapon choices add another loadout decision.
What is not confirmed yet
Mid-mission aircraft changes, resupply rules and restrictions for individual sorties are not fully documented.