A process for upgrading heavy hydrocarbonaceous feedstocks in at least one hydroconversion reactor for hydroconversion of the heavy hydrocarbonaceous feedstocks and in at least one hydrotreatment reactor for hydrotreatment of the heavy hydrocarbonaceous feedstocks, comprising the preparation of two or more catalysts, each catalyst being prepared from one or more catalyst precursor in at least one specific preparation reactor, the catalyst precursor containing at least one transition metal selected from group IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB or IIB of the periodic table of elements, and each preparation reactor feeding one or more hydroconversion or hydrotreatment reactor, each catalyst contained in preparation reactors being dedicated to hydroconversion or hydrotreatment of the feedstocks.