Once you make all that internal / external re-work on the stamped vessel, you have voided the U-stamp and turned it into a second hand equipment. You have to go through a complex testing process to prove that the vessel is good as the new one, then it might retain the value;- otherwise, depending on the modifications you made, the value might go down or may even increase (say you may have deposited gold weld overlay on the previous CRA cladding joints or replaced sections of CRA overlay with an exotic SS material or you have added two more manways and the missing saddle...).
However, if part of the vessel fabrication is the site erection and welding together of the bits and pieces delivered separately on site, that is just normal part of the fabrication, provided that all the relevant code requirements for U-stamping of the vessel have been complied with. In that case, the U-stamp is not voided (because the Vendor has not yet applied the U-stamp) and the vessel value has not been altered since the final value has not yet been established until after the U-stamp and full acceptance.
Is that clear now?
Cheers,
gr2vessels