Reading the AAM documentation I understand that changing a trait rule does not affect the population that was already in that trait. For example, if I set a trait rule: var_1 == 'x' and after a week I change it to var_1 == 'y'
Then users that were already inside with value 'x' will not be suppressed from the trait and will remain there (until the expiration in days specified under advanced options)
But what happens on segments if I change the Segment rule? for example, segment rule is based on trait id = 11223344 and after a week I change the segment rule to be based on a new trait id = 55667788
Will it immediately exclude everyone that was originally in that segment? Will the segment population will drop to 0 until the new trait will start populating?
To answer your question, no, the segment will not exclude everyone that was originally in the segment after the update. However, the segment will begin to qualify users based on the new rule, from the day that the segment rule was updated and modify the population accordingly.
That's why the recommended approach is to always build out a new segment instead of modifying an existing one to eliminate these ambiguities.