Well, Scrooge changed his ways as a result of a "dream" . Does that mean his future "changed"? Does a future that never happens exist? AFAIK there is no experiment to test for the existence of an alternative future which is an excellent reason to ignore the question.
Despite the obvious lack of communication from the last spirit (presumably to beef up the (short) story a bit) I think it's fairly clear that the future would be changed if Scrooge changed his behaviour. As it was he was on course to die a friendless man. But when he changed his ways his popularity went through the roof and so it would obviously change.
I think they are supposed to be ghosts - it's a story.
Not worth getting into the past/future thing too much as you end up wondering what would happen if you went back in time and killed your father.
One of Dickens' driving forces (aside from making a good living as a writer) was encouraging social reform and highlighting brutality and inequality.
His aim here was not a venture into metaphysics but to make people realise that they are not 'islands' without impact upon others; that everyone is part of humanity with influence upon it for the good of all.
Marley warmed Scrooge that he would be visited by ghosts who would attempt to prevent him suffering Marley's own fate. The fact that Scrooge has power to influence the future of himself and others around him is implicit. Of course, he does change, with consequences for Tiny Tim, Bob Cratchit and family and himself that we are aware of, and probably upon others also.
however I do like the idea of a Christmas fund....lay away your Christmas Turkey today.. This was how poor people got their Christmas dinner and geese were plentiful and cheap. I have never eaten a goose, but I do like duck.
And Dickens according to the movie The Man who Saved Christmas with Dan Stevens, Mrs. Dickens was not a frugil woman, not to mention she was very fertile. I love the idea that the story was influenced by Irish paganism.