[RPG] Magic weapon that can change into another weapon

Page 237 of the Rules Compendium states:

Minor Actions

Draw or sheathe a weapon

So to answer your question, your Elven Cleric would have to take two Minor actions; 1 to sheathe her sword and another to draw her bow (with a Free action to load it.)

On a related side note: your Elven Cleric could take the Master at Arms feat (Heroes of the Fallen Lands page 316) which allows the PC to use a minor action to both sheathe a weapon and draw another one.

Long story short, you cannot. All ways to "modify" it will also involve permanently removing all magic from the item.

Most magic dispelling effects are temporary. A Disjunction spell will destroy the item completely. And breaking it until all hit points are lost will remove all magic, but the item can be fixed with spells and remain a simple masterwork item, meaning that it's much easier to simply buy another masterwork item.

As for mundane items, you will have to ask your GM, as there are no rules for this on the Craft skill. And most modifications will be a case by case analysis.

Like, attaching a chain to a kama will make a kasurigama, attaching another axe head to a two-handed axe will make a double-axe. If you make a pointy tip with metal, bone or otherwise and tie it on top of a quarterstaff, you could call it a spear. And so on.

But a lot of items will simply not work that way, and each GM/table will have different opinions on each item individually. Like, what is the difference between a composite bow +1 and a composite bow +4? What makes one stronger than the other?

I would say "everything", the craftmanship is completely different, you need a different wood, a different technique, etc. But some other guy might understand more about crafting of bows and say "no, all you have to do is work on it for a little longer and it will be studier". So this will suffer from table variation.

This kind of modification could easily fall under the Repair Item rules, as the base item isn't changed and is simply being modified, and as such:

• Pay 1/5 of the cost of that new item.
• Make a craft check against the same DC to create the new item.
• If you fail by 5 or more, the cost paid in materials is wasted and you must try again.

You might consider the difference between the two items, if they are too different (new item is more expensive than the base item), you might consider that you must make a new item altogether, and use the base item cost as material cost instead.