I haven't as yet seen any mention of any airline seeking a customisation to their A380's which is in any way unusual; in the past we have seen airlines seeking to move galleys to the cargo deck, in order to free up main deck space; I believe this was on the DC10, but don't quote me on it; plus other major mods. At present all the A380 customers seem content with two full length decks for passengers. As long as the floors can cope with the seating layouts, and the IFE wiring is in the right place there should be few problems. Planes are built to cope with layout changes over the years; look at BA
's 772's, these will shortly gain their 3rd Club World layout, with some planes being under 10 years old. Of course in this case this does depend on a solution being found to anchoring the seats properly !!!!
Before anyone tears me off a strip over the BA
772 club world anchorage problems; I'm sure a solution has been found, or is near to being.