You can promote them once you have acquired the technology needed to have the unit. For example, you cannot upgrade warriors to swordsmen until you have:
1. Researched Iron Working
2. Obtained enough iron
3. Have enough gold
Thanks for replying, and I understand that.
What I'm asking is the ARCHER unit can get PROMOTION_MARCH
<Row>
<PromotionType>PROMOTION_MARCH</PromotionType>
<UnitCombatType>UNITCOMBAT_ARCHER</UnitCombatType>
</Row>
MARCH requires prerequisites
<PromotionPrereqOr1>PROMOTION_SHOCK_3</PromotionPrereqOr1>
<PromotionPrereqOr2>PROMOTION_DRILL_3</PromotionPrereqOr2>
<PromotionPrereqOr3>PROMOTION_ACCURACY_2</PromotionPrereqOr3>
<PromotionPrereqOr4>PROMOTION_BARRAGE_2</PromotionPrereqOr4>
But, ARCHERS can't get the prerequisites
<PromotionPrereqOr1>PROMOTION_SHOCK_3</PromotionPrereqOr1>
<PromotionPrereqOr2>PROMOTION_DRILL_3</PromotionPrereqOr2>
Is only one of these prerequisites required to promote the unit to march?