In general, it is possible to alter a trumpet or fit and flare wedding dress into a sheath dress, but the extent of the alterations required will depend on the specific design and construction of the dress. Here are some factors to consider:

1. Silhouette: A trumpet or fit and flare dress is designed to have a flared skirt that gradually narrows towards the hemline, whereas a sheath dress is straight and narrow from top to bottom. To turn a trumpet or fit and flare dress into a sheath, the skirt will need to be significantly narrowed or even removed altogether.
2. Fabric: The type of fabric used in the dress will also affect how easy it is to alter. If the dress has a lot of intricate beading or lace, it may be more difficult to remove the excess fabric without affecting the overall design. Similarly, if the dress has a lot of layers or a heavy train, it may be challenging to make it into a sheath dress without losing some of the volume and movement.