The change that Scrooge sees in The Ghost of Christmas Present is its aging and dying, for the narrator tells us
"the Ghost grew older, clearly older,"
and that Scrooge notices
"that its hair was gray."
Scrooge asks the Ghost,
"Are spirits' lives so short?"
"My life upon this globe, is very brief," replied the Ghost. "It ends to-night"
What the Ghost is saying is that like his "more that eighteen hundred" other brothers who died before him and who represented Christmas, he too will symbolically die this Christmas night of 1843 (the year Dickens wrote the novella), at "midnight" which is the beginning of a new day.
The Ghost of Christmas Present's death symbolically makes way for his new brother, the next Ghost of Christmas Present; this idea follows the traditional idea of Father Time, who is characterized as a new-born baby at the beginning of a new year, and is an elderly man at the end of the year, ready to die.
The present only lasts for one day, so the Ghost of Christmas Present ages throughout the day.
When Scrooge first meets the spirit, he is young and vibrant. He has “dark brown curls” falling free. It has a “sparkling eye” and a “cheerful voice” and is just generally youthful.
It was clothed in one simple deep green robe, or mantle, bordered with white fur. This garment hung so loosely on the figure, that its capacious breast was bare, as if disdaining to be warded or concealed by any artifice. (ch 3, p. 29)
The spirit’s “joyful air” is in sharp contrast to Scrooge’s demeanor, just as his copious feast has never before been seen in Scrooge’s house.
However, since the present is one day, the ghost gets older as they travel. When the ghost says he has more than eighteen hundred brothers and sisters (remember, the story was written in 1843), Scrooge says this is a “tremendous family to provide for” (p. 29). This is more proof that each “Christmas present” is in fact one Christmas, or one day.
As they travel, the spirit changes. He becomes less youthful. Scrooge does not comment on the change much until the end.
It was strange, too, that while Scrooge remained unaltered in his outward form, the Ghost grew older, clearly older. (ch 3, p. 39)
Near the end of the night, “he noticed that its hair was gray” (p. 39). He asks if his life is so short, and the spirit answers that his “life upon this globe is very brief,” and it ends that night. At this point, Scrooge notices Ignorance and Want, the two wretched children hiding under the Ghost of Christmas Present’s robe. He is horrified. The clocks strikes, and the ghost disappears.