[SuperSajuuk]

I'm curious, what do you think would be the reason? I can't really think of any reason why they couldn't use the original Millionaire theme, unless I'm being completely oblivious to something.

Neil Jones explained it fairly well, but also, these very-early series of Millionaire were heavily sponsored by British companies [The Sun, McVities and so on]. This meant that a lot of the sections which featured these promotions needed to be deleted from the programme and replaced with something that didn't look too out of place or zoomed in significantly to cut them off. The end-title screen was as Neil described: there was no way it could be used, especially since you couldn't both delete out Chris' vocally saying what was on screen and simultaneously cover up the on-screen elements, without blatantly drawing attention to the fact its been edited.

For that reason, replacing them with their own creations that also happened to use the Rave music was fine. Remember, these re-airings came around in 2016, when all people would have likely had access to was the rave music. The turn-around time was tight on it as well iirc, as they had 4 series of episodes that they needed to QA check to see if it was broadcastable, edit out the bits that needed editing, designing a graphic to cover up on-screen elements where possible, but also, finding sequences that could be used as replacement footage for other parts that couldn't be covered up with a graphic. This ended up leading to one instance of a phone number not being deleted out as it should've been [which I think was fixed in subsequent re-airings].

Overall, they did a good enough job, and considering the long history of Challenge having only episodes that were vastly butchered with edits, the work that they did do to make a broadcastable episode that wasn't flooded in edits, in order to maintain the original broadcast length, is pretty good.