Many of the Chorales that Bach used were well-known liturgical tunes and Bach incorporated them for that very reason. He also uses the "Passion Chorale" in the "Christmas Oratorio" in at least two different settings.

It is quite a normal practise for hymn tunes to be used for different sets of words and vice versa. There are many choral works which use existing hymn settings at least as a basis ie "Praise to the Holiest" in "The Dream of Gerontius".

Incidently Keith Marsden borrowed a chunk of Vivaldi's "Four Seasons" for the verse of his song "The Drovers" and Bill Caddick borrowed the melody for "John of Dreams" from Tchaikovsky, who in turn had borrowed it from a southern Italian lullabye entitled Piva Piva.