Dear @Salur.Basbug,
We generally recommend that OpenFAST models of floating offshore wind turbines be defined with zero mean heave displacement. So, if you change the nacelle mass, you should also changing the displaced volume of the floater (PtfmVol0) or mooring pretension accordingly.
I agree that “small” changes in displaced volume likely don’t require a change to the WAMIT solution, but what constitutes “small” is likely subject of interpretation.
Best regards,