Depending on the draconic ancestry of a dragonborn, their breath weapon either deals damage in a 5 by 30 ft. line or a 15 ft. cone.
Would there be any unbalancing if the character were allowed to switch between those two types of AOE? Looking at the Brass and Red ancestries, fire is able to do either, so if it is balanced, why should it not be allowed?
Note: I am asking about changing the AOE only, not the entire ancestry. Also, to clarify, being able to either decide to use it in a line or in a cone at will, or being able to change the area of effect type after a rest.