I would think a change of Nunh would be of benefit - you wouldn't want to mate with your grandpa, for example.

Also, there is possibly other sections of the U tribe around the globe - not just the faction we see in Southern Thanalan. So, it could be very possible for tribe members to migrate from one group to another.

For example, my character is a nunh of the Viper tribe, who migrated to Eorzea as tia to primarily establish a better territory for the L tribe