(To the tune of “Boggis, Bunce, and Bean”)
DS9 s2e19 “Blood Oath” featuring returning TOS actors William Campbell (Koloth, s2e15 “The Trouble with Tribbles,” also played Trelane), Michael Ansara (Kang, s3e7 “Day of the Dove”), and John Colicos (Kor, s1e26 “Errand of Mercy”)
There is no official explanation for their change back in appearance between shows. Roddenberry’s position was to “imagine” the ridges were always there in TOS (make-up of the time being a constraint), but Worf in DS9 renders that point moot.