If the conversion involves the track gauge being narrowed, the existing sleepers can often still be used, but if the gauge is being increased, the sleepers used for the narrower gauge may be too short and may need to be replaced with longer ones.
If the conversion involves the track gauge being narrowed, the existing sleepers, also called rail ties, can often still be used. However, if the gauge is being increased the sleepers used for the narrower gauge may be too short and will often need to be replaced with longer ones.