Warner Bros. Classics & The Great Gremlins Adventure closed in mid-2001 & Scooby-Doo Spooky Coaster opened on June 17 2002. So Scooby-Doo Spooky Coaster's installation took no longer than about a year.
This photo shows that theming and track installation were happening roughly around the same time however it's fair to say the track was installed before theming was added. This photo shows track installation took no longer than 4 months (February to June 2002) while this photo and this photo show the 'Spooky Castle' exterior theming alone took no more than a month and a half (Early April to late May 2002). All this should give an idea of how long it took to create/install the overall theming, though the duration depends on how many people were helping install the ride.
General Comment
Whilst I understand people's disappointment due to their exaggerated marketing, I'm amazed that so many theme park enthusiasts thought the ride would be completely new from start to finish when firstly, the exterior 'Spooky Castle' theming remained (that should imply the queue would be similar & the ghost train section would be fairly similar. After all, it is based on the movie - there were some parts of the ghost train they could add (such 'The Dinner Show') but most of the ghost train section from the movie was already there so there would be minimum change) and secondly, the trains were the same (they were in the new TV commercials too), which meant the ghost train section would be there, which I just detailed. Otherwise, why have 'Spooky Castle' and the same trains if they weren't going to travel through the ghost train section of the original ride? They were never going to stray from the movie's 'Spooky Castle' plot device (not to mention a fully-projection mapped ride would look unconvincing after a while) and I tried warning fellow forum members of this 2 years ago:
It would be very difficult to do a Scooby Doo re-theme - it has to be based around Spooky Castle as that is the entrance to the ride and the carriages are the same as Spooky Castle's carriages. Unless MW changes the carriages (or re-themes them - I don't know how easy that is), it has to be themed to Spooky Castle.
So naturally, it would be the mostly-the-same 'ghost train' section with a few technical changes - mostly the touted projection mapping, which so far it is not and if it is, not applied well. Those technical changes though I'm waiting until they finish 'technical rehearsals' before making a final opinion on them. But one change that can be deemed 'finished' (in a way) is a change in narrative. The original ride's narrative was just trying to escape monsters inside the 'Spooky Castle' ghost train. You then try to find a scared Scooby-Doo who has escaped up into a disco up on ground level in the 'Spooky Island' theme park. After you escaped the 'ghost train' though and found Scooby-Doo, you 'fall' back into the 'Spooky Castle' ghost train but quickly find a safe way out to go back into the 'Spooky Island' theme park.
My theory (due to technical rehearsals still ongoing) on the narrative of the new edition is below: