Se você transformar uma arma mágica em sua arma de pacto, toda vez que você criar sua arma de pacto, sua arma "ligada" aparecerá.
Pact of the Blade (PHB p. 107-108)
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.
Isso também é abordado nas respostas das regras oficiais de abril de 2016 :
If a warlock uses Pact of the Blade to bond with a magic weapon, does that weapon have to be a melee weapon, and can the warlock change the weapon’s form?
You can also use Pact of the Blade to bond with a magic weapon, turning it into your pact weapon. This magic weapon doesn’t have to be a melee weapon, so you could use the feature on a +1 longbow, for instance. Once the bond is formed, the magic weapon appears whenever you call your pact weapon to you, and the intent is that you can’t change the magic weapon’s form when it appears.