The permanent magnetism of a vessel can indeed change in strength due to significant structural repairs. Such repairs may involve altering the materials of the ship, which can inadvertently affect its magnetic field. When steel structures are modified or replaced, the vessel's magnetic field can be influenced, leading to changes in its permanent magnetism.
On the other hand, while the nature of the cargo and changes in heading are relevant considerations for a vessel's magnetic environment, they do not directly cause a change in the vessel's permanent magnetism itself. Cargo can affect a ship's magnetic field temporarily, and the heading can influence how the ship interacts with the Earth's magnetic field, but these are not in the same category as structural alterations. Thus, structural changes are the primary, definitive factor in altering the permanent magnetism of a vessel.