If you multiclass into a class archtype and the archtype replaces an ability for one you previously have, can you keep the base ability instead? For instance, my Level 1 Magus decided the Magus life was not for him and at level 2 I switched to alchemist with the grenadier archtype. The Grenadier archetype replaces the brew potion feat with a proficiency in a specific martial weapon. My one level in Magus means I am already proficient in all martial weapons, so do I take the brew potion feat, or do I just get nothing? Or is it a GM call?