I've got a character in my campaign who was lucky enough to snag a Rod of Lordly Might.
The snag is that she's a Hexblade Warlock, and she wants to use the Hex Warrior ability on the rod (basically get her CHA bonus on attack rolls)
If you press button 1, the rod becomes a Flame Tongue as a fiery blade sprouts from the end opposite the rod's flanged head.
If you press button 2, the rod's flanged head folds down and two crescent-shaped blades spring out, transforming the rod into a magic Battleaxe that grants a +3 bonus to Attack and Damage Rolls made with it.
If you press button 3, the rod's flanged head folds down, a spear point springs from the rod's tip, and the rod's handle lengthens into a 6-foot haft, transforming the rod into a magic spear that grants a+3 bonus to Attack and Damage Rolls made with it.
My answer was, "No. It's a rod, not a weapon."
Her counter was, "What if I activate a form of the rod, then attune?"
My return answer was... "Uh, let me think about it."
My gut says, "Allow this, but as soon as the form of the rod changes, it's no longer a pact weapon."
But really the question is what's RAW?
(and yeah, I looked at Can the Rod of Pact Keeper +1 be selected as a pact weapon by a Pact of the Blade warlock?. Not exactly the same. This is a basic question of the primary Hex Warrior feature - XGE: page 55)