Guards become guard captains, and become military leaders, protectors of nations, slayers of mighty beasts, or even kings. I think that's a well established archetype in most fantasy literature.
Also, fighters are not guards, they are already considered more talented, or possessing something on top of a regular soldier as established by the fiction in the PHB:
Trained for Danger
Not every member of the city watch, the village militia, or the queen’s army is a fighter. Most of these troops are relatively untrained soldiers with only the most basic combat knowledge. Veteran soldiers, military officers, trained bodyguards, dedicated knights, and similar figures are fighters.