Well, technically, there were three overall releases of each game.
1. The floppy-disk version. (X-wing/Imperial Pursuit/B-Wing; Tie Fighter/Defender of the Empire)
2. The CD-Rom version. (X-Wing Collectors CD-ROM edition; Tie Fighter Collector's CD-ROM edition). This edition did make graphical enhancements to X-wing (added gouraud shading, altered some cut-scenes, voiceovers for mission briefings, etc.). For Tie Fighter, it included the final expansion, SVGA graphics (so, you could run in 640x480 res), some digitized cut scenes, and the Enemies of the Empire expansion.
3. The X-Wing Collector's Series Edition boxed set, which included X-Wing, Tie Fighter, and an abridged version of X-Wing vs. Tie Fighter (XvT). All of this was done on the XvT engine, which used texture mapping instead of gouraud shading, made all of the games in higher-res graphics, but otherwise retained most of the things from the originals. What it got rid of, though, was some of the launch sequence animations, assigning friendly pilots to your squadron, and a few other minor things. This version was compatible with Win95/98.
It's #1 and #3 that are being released, rather than #2 and #3, which is (I think) Jeyl's point: why release the original disc version when the CD-Rom version already had that and some other improvements.