As far as I understand Aerofoil was asking regarding quick-change or convertables, not combis.
I never heard about 747 quick change (like 737QC), allowing to switch all-cargo to all-pax within an hour, using pallet-mounted seats and interior.
But there are convertable 747s, equipped with nose cargo door. And yes, they can be converted from pax to all-cargo and vice versa within few days (say 2-3 days). Martinair of Holland operating 2 such planes and ACTUALLY doing convertion depending of pax/cargo demand.