Metaphoric Metamorphosis

In cartoons, characters will sometimes change into something depending on the situation:

A lollipop emblazoned with the word "Sucker"
A donkey, with optional braying
Shoe heel (this example is often aligned with guilt and remorse akin to the trope My God, What Have I Done? rather than being embarrassed.)
A dumbbell
A wolf, complete with Wolf Whistle

Examples of Metaphoric Metamorphosis include:

Live Action TV

Used in iCarly: Spenser and Sam are the last ones surviving in the assassination game, with Spenser becoming increasingly paranoid. A deliveryman delivers to Spenser's apartment what Spenser calls a huge lollipop. The deliveryman says, "Yeah, but where I come from, we call it a... SUCKER!" He dives out of the way, and Sam nails Spenser from the doorway.

Web Comics

Mort in Gunnerkrigg Court did transform into a rather descriptive tombstone—with an added bonus that he's quite able to pull this In-Universe.

Western Animation

Very common in Looney Tunes
- In "The Big Snooze", Elmer Fudd turns into a sucker after repeatedly falling for Bugs' "run through a hollow log over the edge of a cliff" gag.
  - This episode is basically a retrace of a scene from the earlier short "All This And Rabbit's Stew", with the black caricature hunter redrawn as the similar proportioned (but more politically correct) Elmer Fudd.
- Happens in "The Million Hare", after Daffy inadvertently donates his million dollar prize to his "best buddy" Bugs. The touched announcer asks Daffy for some words, to which a "ass-faced" Daffy donkey-brays.
- In the short "Tom Turk and Daffy", Porky Pig first turns into a Dunce Cap, then a sucker, and then a bottle of dope.
Used a few times in The Flintstones as well. Played with in one example when Fred is guilt tripped by Wilma. He runs into Barney, who comments on his own still visible "heel" head, implying Betty used the same tactic on him.
Pretty common in Tom and Jerry, often after Tom tricks Spike the Bulldog into chasing after a bone.
There are two Classic Disney Shorts in which characters turn into heels: "The Sleep Walker" (in which Pluto destroys a dachshund's dog house, leaving her and her puppies without shelter), and "Cured Duck" (in which Donald gets so frustrated from trying to open Daisy's window that he wrecks her house).
In Ren and Stimpy, Ren turns into a man's ass, accompanied by a donkey braying.
An episode of SpongeBob SquarePants has Squidward's head be replaced by that of a donkey. Or, more precisely, an ass.