At heart, it's almost the Ship of Theseus paradox. By that logic, you can call into question whether all the ancient woodies around the world are separate credits and if so, at what point they can be considered to be 'new'. Personally, I don't have a hard and fast rule, I'd do it on a case-by-case basis. Revolution at Blackpool might be a different colour and have a different name to when I rode it first, but I don't count that as new, I wouldn't count Walking Dead/X or pre- and post-loop SoB as new but I would count RMC conversions and relocations. Colossos for me isn't new - the machine is fundamentally the same, in the same place and I don't think the experience will be substantially different. It's extensive maintenance, sure, but it all comes back to the Ship of Theseus - you can replace a whole bunch of parts until every last original atom is gone, but what constitutes the ship (or in this case, Intamin prefab wooden rollercoaster) itself is the greater object as a whole, so it's valid to argue that's what matters.