A fixed bed hydroprocessing system, and also a method for upgrading a pre-existing fixed bed hydroprocessing system, involves preliminarily upgrading a heavy oil feedstock in one or more slurry phase reactors using a colloidal or molecular catalyst and then further hydroprocessing the upgraded feedstock within one or more fixed bed reactors using a porous supported catalyst. The colloidal or molecular catalyst is formed by intimately mixing a catalyst precursor composition into a heavy oil feedstock and raising the temperature of the feedstock to above the decomposition temperature of the precursor composition to form the colloidal or molecular catalyst in situ. Asphaltene or other hydrocarbon molecules otherwise too large to diffuse into the pores of the fixed bed catalyst can be upgraded by the colloidal or molecular catalyst. One or more slurry phase reactors may be built and positioned upstream from one or more fixed bed reactors of a pre-existing fixed bed system and/or converted from one or more pre-existing fixed bed reactors.
