While Goku has possessed the power to gather energy to form the devastating Spirit Bomb, Vegeta learned an opposite technique in the Dragon Ball Super manga series: the powerful Forced Spirit Fission. Giving Vegeta mastery over raw energy and making him able to diffuse and redirect collected power, this new technique is one of the most unique abilities seen in the Dragon Ball franchise to date. And as Vegeta further develops his familiarity with Forced Spirit Fission, this technique may be able to undo a mistake made millions of years ago involving the Elder Kai and his own forced fusion.
Vegeta learned Forced Spirit Fission while training on the planet Yardrat, the faraway world where Goku learned Instant Transmission. Determined to significantly augment his combat power and learn new techniques to learn the magical supervillain Moro, Vegeta endured some especially rigorous training. Returning to Earth for the final showdown against Moro, Vegeta used Forced Spirit Fission to strip the energy Moro stole from New Namek and its inhabitants, restoring them to life as a means to atone for his own attack on Namek for the Dragon Balls during Dragon Ball Z. As Vegeta explains how his new technique works, he reveals that Forced Spirit Fission is capable of far more than simply draining and redirecting raw energy.
Vegeta claims that Forced Spirit Fission is so potent that it can undo fusions. He elaborates on this by claiming he could undo Piccolo's fusions with Nail and Kami if he wanted to, even as long-lasting and seemingly permanent as these fusions are.
With this in mind, Vegeta could theoretically undo the Potara-fueled fusion between the Elder Kai and the old witch he had accidentally been fused for millions of years before being imprisoned in the Z Sword by Beerus. While Vegeta has yet to demonstrate using Forced Spirit Fission to undo a fusion, he has given no cause to doubt that he is capable of doing so, and he's already proven himself to be adept at this new technique during his rematch against Moro.
Defusing the Elder Kai from the witch would theoretically make the one-time Supreme Kai noticeably younger, with the Elder Kai hinting that his accidental fusion led to his physically older appearance. While Kais, specifically with Gowasu from Universe 10, have been shown to age, they do so at a considerably slower rate than mortals.
Any magical abilities that the Elder Kai may have gained from this fusion would be lost, but the Kai would likely be happier to be intact in his own body, with his individuality restored. Although the Elder Kai expressed disapproval in the Dragon Balls tampering with the natural laws of the universe -- a distinction not shared by Universe 7's current Supreme Kai who used them to defuse from Kibito -- Forced Spirit Fission is a different matter entirely.
The Elder Kai is one of the more tragic characters introduced in DBZ, imprisoned within a mythical sword for millions of years after undergoing an accidental, permanent fusion with a nosy witch. The Elder Kai has accepted much of the humiliation and injustice that has come his way, but Forced Spirit Fission could offer him the means to finally reverse one of the stranger fates that he has been forced to endure over his long life. However, the Kais tend to get the short end of the stick when it comes to their treatment in the Dragon Ball universe, and the Elder Kai's prolonged forced fusion appears to be no different.