RIght now, when planets trade hands, all population is killed by default.
This should be a per-planet option, with the default being that no population is killed. When choosing to depopulate a planet, a player should be able to reduce to a percentage, or to a specific amount.
This would address this problem where intra-fam passing hurts the fam’s economy. It will also allow a player to receive more value from taking an enemy’s planet if they get to retain its population.
By making this an option, a player will also have more control to manage their own population if/when they ever have reasons to drop their own pop.
This should also support mass-planet de-population so players can easily perform mass-genocide on captured enemy planets if they choose.