I was reading a recent question about Warlock Pact Weapons and Attunement, and it got me thinking. The answers there indicate "A Magic Weapon that is also a Pact Weapon still requires attunement." However, although the answers there don't seem to say so explicitly, my understanding is that the intent is less "Requires attunement" and more "does not remove attunement requirement" or "does not count as attuned without using an attunement slot".

If a Warlock has a Magic Weapon, but is not attuned to it, can they make it their Pact Weapon and gain the normal features granted by Pact Weapon?

Yes, an unattuned magic weapon can be a pact weapon.

While not attuned to a magic weapon requiring attunement, you don't benefit from its magical properties... but that doesn't suspend its classification as a magic weapon.

Here are the relevant rules for Attunement from the Basic Rules:

Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it.... Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise.

However, nothing in that rule indicates that the magic weapon stops being a magic weapon while you're not attuned to it, only that you can't benefit from its magical properties during that time. Being a "magic weapon" isn't really a property, per se. It's a categorical description of the intrinsic nature of the weapon.

Now refer to the rules of the Pact of the Blade feature:

You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon.

There's no stipulation other than that the magic weapon you turn into your pact weapon must be... a magic weapon. It still is, even when you're not attuned to it.

Note that the unattuned magic weapon as a pact weapon still benefits from being treated as magic according to the Pact Boon feature simply because it is a pact weapon (any pact weapon is treated as such whether it is a magic weapon or not), much less because it is still a magic weapon:

This [pact weapon] counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

You betcha!*

In order to a magic weapon a pact weapon, the Pact of the Blade states:

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. You can’t affect an artifact or a sentient weapon in this way.

There isn't any requirement that magic weapons that require attunement must be attuned in this pact weapon language. But that doesn't mean you get all of the whiz-bang properties of that magic item.

*You can use it, but it won't have magical properties

Page 138 of the DMG also explicitly states:

Without becoming attuned to an item that requires attunement a creature gains only its nonmagical benefits.

Ultimately, you can definitely make an unattuned magic item your pact weapon if you don't attune to it - but you won't get any of it's magical properties (bonuses to hit, etc.)

*Attacks will still be magical damage

However, per the language of the Pact Weapon, the attacks will still be magical.