I personally think button is unnecessary. The most people will decide if they want to use lbs or kgs just once in the beginning (maybe it would be ok to create

I personally think button is unnecessary. The most people will decide if they want to use lbs or kgs just once in the beginning (maybe it would be ok to create view for choosing right after registration) and this decision is based on metric system which is used in their region. If they want to change it at some point, it'd be still possible to do so in their profile settings.