Shifting weapon rune (Blade Ally)

[Prethen]

I'm not sure what "similar form" means. Can I shift my Greatsword into a Great Club, so I can change the type of weapon damage from slashing to bludgeoning? I'm curious as to what benefits this power has otherwise.

[Taja the Barbarian]

That phrase is part of the 'fluff' description and doesn't refer to actual rules or game terms.

The key restriction is in the actual ability text

Shifting Rune wrote:

Magical, Transmutation

Source Core Rulebook pg. 585
Price 225 gp
Usage etched onto a melee weapon

With a moment of manipulation, you can shift this weapon into a different weapon with a similar form.

Activate Single Action Interact; Effect The weapon takes the shape of another melee weapon that requires the same number of hands to wield. The weapon’s runes and any precious material it’s made of apply to the weapon’s new shape. Any property runes that can’t apply to the new form are suppressed until the item takes a shape to which they can apply.

So, you can change your Greatsword into any other melee weapon that requires two hands with this rune.