From Ruud Altenburg's dicography http://www.illustrated-db-discography.nl/ :

John I'm Only Dancing (Bowie): a number of versions exists; the 1974 Young Americans version is discussed below as 'John I'm Only Dancing (Again)'. The original version of this song was released as a single in September 1972 (RCA 2263). In April 1973, it replaced by a completely different re-recording from the Aladdin Sane sessions ('sax version', confusingly also RCA 2263). Both appeared on the 1976 ChangesOneBowie LP, the sax version albeit erroneously, only on the first 1000 UK pressings. On most editions of the The Best Of Bowie compilation, the sax version can be found (but on e.g. the Italian issue the original version is included). In 1979, a slight remix of the original version was released either as B-side of the 1974 version (UK, RCA BOW 4), or as A-side (US, RCA PD 11887). Rykodisc has re-released all three versions: the original version has appeared on ChangesBowie and the Singles Collection; the sax version on Sound + Vision I and The Best Of David Bowie 1969/1974; and the 1979 remix as an extra track on the re-release of The Rise And Fall Of Ziggy Stardust And The Spiders From Mars. A 2'51" video clip of the original version is featured on The Video Collection. Besides that, live versions are issued on the Sound + Vision Plus CD-single and on Santa Monica '72.

In other words,the 'previously unreleased' version from the Ryko Ziggy was released in 1979 by RCA.