What? How did you change it? With an AssumeFPS? If so, bad move. When one talks about changing from 29.97fps to 23.976fps, they usually mean performing an IVTC.
If that's what you're trying to do for a DVD source you first make the D2V project file using 'Honor Pulldown Flags' as the Field Operation, followed by opening the video using MPEG2Source and then do the IVTC using something such as:
TFM().TDecimate()
The framerate changes from 29.97fps to 23.976fps, the length remains the same and the audio remains in synch. If your audio went out of synch you did something different.
As usual, if you're asking for help on what to do with a DVD source you have, cut a small piece using DGIndex and upload it here.
As the result of an IVTC the framecount is cut by 20%, the framerate is cut by 20%, and the length remains the same.But found that confusing as to how that's possible