The Swoof formula is a formula that was used by the Smurfs in the comic book and cartoon show versions of "The Astro Smurf" (and presumably also in "Dreamy's Pen Pals") which was used to transform the Smurfs into Swoofs. In the cartoon show episode "Good Neighbor Smurf", Brainy accidentally recreated the formula while he was sleeping over at Papa Smurf's house, causing Papa Smurf to transform into Grandpa Swoof.
Empath: The Luckiest Smurf stories[]
In the EMPATH: The Luckiest Smurf story series, the "reality bubble" spell was used in place of the Swoof formula to change the Smurfs into Swoofs the second time that Dreamy wanted to visit the Swoofs.