The strength of the upgraded structure depends mainly on the yield strength of the material in it, how the geometrical features of the structure are changed, and on how the loads are introduced into the new structure. If the geometry of the upgraded structure remains unchanged and instability or fatigue are not an issue, the strength of the structure will increase proportionally with the increase in yield strength of the material. However, if the aim of the upgrade is to reduce weight by decreasing the thickness of the material, instability and fatigue become more likely and must be carefully considered. In such cases, the strength of the upgraded structure may not increase proportionally with the increase in yield strength, and the effects of instability and fatigue must be considered.