The Pact of the Blade feature of the Warlock reads (emphasis mine):
You can use your action to create a pact weapon in your empty hand. [...] You are proficient with it while you wield it. [...]
It goes on to say:
You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon. [...] ...it appears whenever you create your pact weapon thereafter. [...]
Suppose my human warlock got her hands on a nice +1 warhammer. Normally, she is not proficient with warhammers. If she uses this feature to transform the +1 warhammer into her pact weapon, she can later summon it at will.
Am I correct in interpreting that as long as she wields the +1 warhammer as her pact weapon, she is proficient with it? Or does that clause only apply to the "default" pact weapon that the warlock creates themselves?