Well, are you trying to build this as gear, an Advantage, or something else? For gear, you can basically just say "Hey, this takes
n (seconds/FP/whatever) to change forms," possibly with the ability for a relevant skill to reduce this. For an Advantage,
this might be of use, allowing you to add extra time, an FP cost, etc for changing from one ability (weapon form) in an Alternate Ability set to another. Turn the entire AA set into a single trait, then apply appropriate Gadget modifiers. If you need to price the weapon as gear, a decent way is to build it as a single trait, as above, then then use the rules from
Metatronic Generators (Pyramid #3/46) to turn it into gear.
For something in-between, you could actually build a weaker version using the Pyramid article, then have the appropriate Enhancements as an Advantage the character has. For example, in
Flame of Recca, the Kougan Anki is a weapon with 5 forms - a naginata, a kusarigama, a giant pair of scissors (no, really), a boomerang, and a bow. It's actually a complex mechanical puzzle to change forms - a skilled user can go from one form to the next in, IIRC, 10 minutes or so. You'd build that with Immediate Preparation Required, for the default weapon. The character who uses it in the manga/anime, Koganei, is stated to do it in slightly over 2 seconds, but the way it actually works during fights is that he does it instantly - he'd buy off the Preparation Required.