why you so afraid? I mean as long as that fusion monster has common material, (like Blue-Eyes Ultimate Dragon/ Dark Paladin) this card still can be used. I think fusion monster that needs specific material like Elemental Hero Elements fusion monsters (Escuridao, Gaia, Nova Master), Red-Eyes Slash Dragon or Buster Blader The Dragon Destroyer Swordsman cannot be used with fusion substitute.
It's the same rule as for Polymerization: You can replace any specific NAMED monster but not generic monsters, e.g. you can summon Red-Eyes Slash Dragon and replace Red-Eyes B. Dragon with any Substitute but you cannot replace the Warrior monster because it's not a specific monster.