I sometimes turn a short story into Flash as an exercise. What I try to avoid is ending up with a piece that has lost weight but is still wearing the same old clothes. I focus on a single scene, lose a side-plot, or lose a character. If I return to the short story I'm usually able to exploit what I've learned when writing the Flash.
Sometimes I've made a page-long poem more episodic, then I've broken it into a few poems. Not all of the shorter poems succeed, but at least I've salvaged something.
Welsh writer Cynan Jones' story “The Edge of the Shoal” began as a 30,000-word short novel but he cut it to 11,500 words because “it didn’t work.” When he sent it to The New Yorker they liked it but asked him to cut it in half. He took 4 days to cut the story to 6,000 words. In that form the New Yorker published it and it won The 2017 BBC National Short Story Award. The original version was published by Granta as a novella entitled "Cove", which then won the Wales Book of the Year Fiction Prize.
Moral - you may want to keep more than one version of some of your pieces - short and long versions. If you chop, keep your drafts. You may never become famous enough to sell them, but they may have something valuable that gets worn away by rewrites.