Totally agree Jordi. Anyway, a clear rules to transform national data to EVRF2000 should be written somewhere.
Contour regeneration would not be a problem as far as can be obtained from DTM but the collateral efects are still not known. In the Spanish case, an offset of 49cm must be applied to have EVRS ellevations.
Then, yes. The richness of true 3D vector data of national agencies is lost in order to be consistent with elevation models and that models could have two different transformations: a) the rigorous tilted plane as published in crs.eu or b) a simple elevation offset in case the differences along the country were neglectables