James Doohan who played Scotty (Montgomery Scott), the miraculous engineer who could always get a little more out of the Enterprise appeared on TNG as his original character who has been promoted to captain.
With parallel dimensions and alternate time lines, Denise Crosby returned as a guest star in later TNG episodes as Tasha Yar's offspring and as Tasha from an alternate timeline.
It does not apply if the related union member is in one of the following unions: Unite Here, AFGE, TNG - CWA, AFSCME, TCU, RWDSU, OPCMIA, AFT, BCTGM, CWA, NABET - CWA, CSEA.
DeForest Kelley who played Leonard "Bones" McCoy in the original series made a guest appearance in the first episode of TNG as an admiral of 137 years of age.
He not only made a guest appearance on TNG, but played a recurring role as his original character that even carried over to the latest Star Trek movie.