I am working on a routine where I need an object (in this specific case, a dagger) to transform from one form to another. Most of the transformation effects I know, such as change bag, only work with soft or very small (hide in the hand sized) objects. And effects that can do similar for rigid objects, like the drawer box, square circle, flip-over vanish, or Chinese temple screen, can only produce or vanish, not transform from one to another.

I'm obviously needing to broaden my knowledge base of transformation effects, so I ask what are some of your favorites that I may not have heard of?

Rose petal to egg. A rose petal is placed into a wine glass and rotated and turns into a real egg.

Mirror or Black Art dividing a clear plastic tube into two sections. Simply rotate the tube 180 degrees to effect the switch (ala a Conradi device or a mirror glass).

My "Five Smooth Stones" has been called by some magicians "the most magical" they have seen.

Following the story of David and his sling, six actual stones of different colors are dumped from a bag.
Since only five are required, a spectator selects one to be set aside. The others are returned to the bag and pocket.
The chosen stone is held at the fingertips and tossed into the air to visibly change to a silk of the same color to float to the ground.
Your hands and body are clean.

This could be done with a small dagger, but not sure of a suitable story line. Maybe something based on the game Clue where the murder weapon transforms - 4-6 various weapons to chose from.

I could see chips with the various weapon pictures thereon. The chose dagger chip is tossed in the air to fall to the table and stick therein- maybe 6" long as the chip vanishes.

I have produced a real 14" sword that could be done as a transformation, but is not very angle proof. (Hull Silk vanish in reverse)

Also, consider a change cap.

Some variation on the Bengal net?

https://funtimemagic.com/bengal-net/

When I was a teenager I finished my show with a "goodnight banner". A rectangle of black velvet, with rods stiffening the two short ends, was doubled up into a horizontal receptacle, glitter was poured in and then the banner dropped to show "goodbye" written on the fabric. The method was very similar to a change bag but could easily accomodate a more bulky flattish item. Similar methods have used props made of cardboard. A method using newspaper could be a bit more "natural"

If the transformation is to something of similar size and shape to a dagger I would go for wrapping it in a silk or tissue paper, dumping the dagger and introducing the object, separating the wrapped item from the body,table (or wherever dumped) ideally have a hint of the original dagger showing (can be palmed or dumped at the last minute), allow a few beats of time misfirection, giving a totally clean moment of revelation

But that's just me! Think Topit, Servante, Black Art on the way there.

Chris's offerings make me think of a very simple method based on the old 'table magic' effect of vanishing a table knife.
A dinner napkin is spread on the table and a table knife laid across the center. One edge is folded over and the cloth rolled into a tube with the hands clean.
When the tube is unrolled the knife has vanished.

The sudden spread unroll of the cloth from tube to flat could easily mask a dagger from behind the hand/wrist dropped onto the cloth center.
Any object like a drawing of a knife or word knife on a slip of paper could come first. Normally, sound is a problem with the now hidden knife,
but here, the clunk of the dagger appearing on the cloth could be an advantage.

can't describe the simple 'flap over' method here, but easy to find or write me.

I have used a flip-over box to do a change. The changed object is loose in the box to start. It flies out as the other object vanishes. Be ready to let go of one side to catch the object. I was younger and quicker then.

I have seen the vanishing table knife method mentioned by funsway called the "gypsy switch." This label may make it easier to find.

Abbotts used to make a changing tube with end caps having an oval diagonal partition going from one end to the other. This would be perfect for a dagger because of the blade at one end. Perhaps you could make something like this with a square tube.

The Bengal net is a really good suggestion.

This is/was known as The Casket of Pandora.