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.
The existing sleepers can often still be used. The conversion involves the track gauge. This track gauge is being narrowed. The sleepers used for the narrower gauge may be too short. The sleepers used for the narrower gauge may need to be replaced with longer ones. The gauge is being increased.