How does armor and weapon proficiencies work with sidekicks?

Question

A noble wears a breastplate and wields a rapier, but the "Bonus Proficiencies" section of the spellcaster sidekick class reads in part:

The sidekick gains proficiency with light armor, and if it is a humanoid or has a simple or martial weapon in its stat block, it also gains proficiency with all simple weapons.

Does this mean that the sidekick is no longer proficient with breastplate armor and rapiers?

The answer to a possibly related question, states that:

Assume that a creature is proficient with its armor, weapons, and tools. If you swap them out, you decide whether the creature is proficient with its new equipment.

Would this rule in the Monster Manual still apply to sidekicks and would that mean that a noble spellcaster sidekick would be proficient with light armor, breastplate armor (specifically, not medium armor), simple weapons, and rapiers?

Answer 1

The noble would retain proficiency with breastplate and rapiers.

The sidekick rules state which proficiencies the character gains. Gaining a proficiencies does not cause you to lose any existing proficiencies.

Unless otherwise stated, the rule granting a noble proficiency with rapiers and breastplate is not overridden. So the rule you quote from the Monster Manual stands. As such, the noble has proficiency with the weapons and armor in its statblock, and in addition, gains proficiency in all light armor and all simple weapons.