You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it. You are proficient with it while you wield it. This weapon counts as magical for the Purpose of overcoming Resistance and immunity to nonmagical attacks and damage.
You can transform one Magic Weapon into your pact weapon by performing a Special ritual while you hold the weapon. You perform the ritual over the course of 1 hour, which can be done during a Short Rest. You can then dismiss the weapon, shunting it into an extradimensional space, and it appears whenever you create your pact weapon thereafter. […]
Say I attune to a Flame Tongue (longsword), is that longsword now the only weapon I can summon through this feature? The Sage Advice makes it clear I cannot reform my Flame Tongue into a javelin, but could I just conjure a regular javelin that is not the Flame Tongue, as I could before I attuned to the magic weapon?