I think the smart money is on the -8I, but not 1 for 1 replacements. CX will probably order between 6 and 12. They already have 10 -8 options which could be firmed as freighters or Intercontinentals. -8Is would be used primarily for slot restricted European airports and seasonally to J-burg due to the high field elevation. The 777 does not work well there due T/O performance, especially in the summer.
Passengers would probably prefer the A380, but I've heard CX is not impressed with its cargo capabilities on long haul routes. The -8I with "sky" interior should be a close second in terms of passenger experience with the biggest difference being the increased cabin noise in the Boeing.