The rotten wood was replaced because it was rotten...that was the reason...H&S.
We haven't got a clue what has happened to the interior, only that there was a can of wd40 in an open doorway, if I recall correctly.
Anything else...we just don't know.
Alton Towers only have an obligation to maintain Health and Safety requirements on things which are actually in use, be that by the resort, the guests or (in the case of a ride, both).
Given the fact that the queue line has been ripped up and replaced with a brand new decking, that’s a fairly clear indication that they intend to use the queue line again, which suggests that the building will house an attraction of sorts.
There’s been numerous photographs of the front doors to the building bro by open with work going on inside, obviously we don’t know what the extent of that work is but it’s certainly more significant that a photo of a rogue can of WD-40 being found.
As I keep saying there’s obviously a chance that all this work is being done to ready the building for something else, as general maintenance etc, but given the fact that they’ve never done this before and they’re now choosing to do it the season that the major rollercoaster in the area is unavailable, my educated guess will be that it’s not a coincidence.