Well if you code the triggers well the transition will be slowly and not immediately if you conquer for example Bactria. First of all I would divide the trait into levels: (please note that the effects are still not finished)
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{Lives_on_the_move} Lives on the move
{Lives_on_the_move_desc}
This man belongs to a nomadic culture. As such he has trouble adapting to a more settled life. He prefers to live on the road, moving from place to place as necessary to fulfill his needs. However this means that he is less inclined to develop a more longterm plan for farming and trading.
{Lives_on_the_move_effects_desc}
+20% Movement, 15% discount on animal unit training costs, 25% bonus to cash gained from looting, +1 Command when attacking, +2 Command when commanding cavalry, -1 to farming output, -10% bonus on tax income, -10% to trade income
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{Nomadic_Nature} Nomadic Nature
{Nomadic_Nature_desc}
This man is a nomad. He doesn't like to settle down anywhere, preferring to move to other places when necessity arises. Because of this lifestyle he is not suited to focus on farming, trading or taxation. When he is on the move, though, he does it with force, having acquired expertise in raiding and commanding his horsemen to attack enemies.
{Nomadic_Nature_effects_desc}
+40% Movement, 30% discount on animal unit training costs, 50% bonus to cash gained from looting, +2 Command when attacking, +3 Command when commanding cavalry, -2 to farming output, -20% bonus on tax income, -20% to trade income
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{Horde_Chief} Horde Chief
{Horde_Chief_desc}
This man is the potential commander of a Horde. Moving around exploiting resources like a wave of locusts, he should never stop anywhere to settle. Being on the offensive is the only thing that motivates him. His abilities shouldn't be wasted on management duties, which he can't fulfill properly anyway. Commanding an army of horsemen, though, he is a terrible force that cannot be ignored.
{Horde_Chief_effects_desc}
+80% Movement, 60% discount on animal unit training costs, 200% bonus to cash gained from looting, +4 Command when attacking, +5 CavalryCommand, -5 to farming output, -50% bonus on tax income, -50% to trade income
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{Has_lived_in_a_town_eastern_civilized} Wants to settle
{Has_lived_in_a_town_eastern_civilized_desc}
Throughout history, groups of steppe nomads, from only a few families within a large nomadic tribe, to clans and to almost whole tribes, have become sedentary for diverse reasons. By coming from one such group, this man has acquired some abilities that make him more suitable to rule over settled peoples than his counterparts living in "the old way". Thanks to his skills, he is likely to benefit farming and trade, but he won't have as much success as his more nomadic peers if he tries to lead a steppe army on campaign.
{Has_lived_in_a_town_eastern_civilized_effects_desc}
+1 to farming output, +5% to trade income, +5% bonus on tax income, +1 Command when commanding infantry, -2 Command when attacking
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{Settled_Nature_eastern_civilized} Settled Nature
{Settled_Nature_eastern_civilized_desc}
By having led a settled life himself for a substantial time, this man is reasonably well versed in matters such as farming and the orderly runnig of a town, that are completely alien to his nomadic peers. Conversely, he cannot compare to them in the skills that the nomads acquire throughout their lives and that are such a great help to lead an army on aggressive campaigns.
{Settled_Nature_eastern_civilized_effects_desc}
+2 to farming output, +10% to trade income, +10% bonus on tax income, +2 Command when commanding infantry, -3 Command when attacking
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{Settled_eastern_civilized} Settled
{Settled_eastern_civilized_desc}
While his forefathers may have been nomads, this man has only been on horseback for sport and has ruled over settled peoples for so long that the skills and knowledge needed for a nomadic life are but a memory to him. However, from such a life he has become expert in the sound use of farming land and finances. His talent should not be wasted by putting him at the head of an army in an offensive campaign; a task for which he lacks any ability.
{Settled_eastern_civilized_effects_desc}
+4 to farming output, +20% to trade income, +20% bonus on tax income, +3 Command when commanding infantry, -6 Command when attacking
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{Has_lived_in_a_town_western_civilized} Wants to settle
{Has_lived_in_a_town_western_civilized_desc}
Throughout history, groups of steppe nomads, from only a few families within a large nomadic tribe, to clans and to almost whole tribes, have become sedentary for diverse reasons. By coming from one such group, this man has acquired some abilities that make him more suitable to rule over settled peoples than his counterparts living in "the old way". Thanks to his skills, he is likely to benefit farming and trade, but he won't have as much success as his more nomadic peers if he tries to lead a steppe army on campaign.
{Has_lived_in_a_town_western_civilized_effects_desc}
+1 to farming output, +5% to trade income, +5% bonus on tax income, +1 Command when commanding infantry, -2 Command when attacking
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{Settled_Nature_western_civilized} Settled Nature
{Settled_Nature_western_civilized_desc}
By having led a settled life himself for a substantial time, this man is reasonably well versed in matters such as farming and the orderly runnig of a town, that are completely alien to his nomadic peers. Conversely, he cannot compare to them in the skills that the nomads acquire throughout their lives and that are such a great help to lead an army on aggressive campaigns.
{Settled_Nature_western_civilized_effects_desc}
+2 to farming output, +10% to trade income, +10% bonus on tax income, +2 Command when commanding infantry, -3 Command when attacking
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{Settled_western_civilized} Settled
{Settled_western_civilized_desc}
While his forefathers may have been nomads, this man has only been on horseback for sport and has ruled over settled peoples for so long that the skills and knowledge needed for a nomadic life are but a memory to him. However, from such a life he has become expert in the sound use of farming land and finances. His talent should not be wasted by putting him at the head of an army in an offensive campaign; a task for which he lacks any ability.
{Settled_western_civilized_effects_desc}
+4 to farming output, +20% to trade income, +20% bonus on tax income, +3 Command when commanding infantry, -6 Command when attacking
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Secondly it would require multiple causes to fully change from nomad to settled, region only isn't enough. And it will only start to happen quickly once the faction leader becomes settled
