I'm not quite sure what you mean with your question. I wrote the following recently in another thread, but maybe you mean something different:
If your old .m3u files are without path information, then they must be in the same folder with your music files before importing to MC. Afterwards you can change the location of your music files by MC and all internal playlists are kept up-to-date.
If there is path information in the .m3u file, it must be correct.
MC reads information from .m3u files and makes internal playlists based on that information.
for example:
Winamp .m3u without path (playlist in the same folder):
#EXTM3U
#EXTINF:-1,The Art of Noise - Love
Art of Noise - Draft - 01 - Love.mp3
#EXTINF:-2, ...etc
Winamp .m3u with path (playlist not in the same folder)
#EXTM3U
#EXTINF:-1,The Art of Noise - Love
D:\Music\Art of Noise\Draft\Art of Noise - Draft - 01 - Love.mp3
#EXTINF:-2, ...etc