You only lock in the amount of weapon skills you need when you trade the mythic nq weapon to the blue mage npc in lower jueno who flags/starts your mythic ws quest. You may acquire any nq mythic weapon at any time and not be penalized for your floor progression, however once you trade said weapon to the ws npc in lower jueno whatever floor you are on at the time is which amount of ws you will need to do to unlock said ws.