the coat usually changes, it can be subtle (white to cream) or it can be something as prominent os vixies changing, when she was bron she was red wiht blue sabling covering her entire body as she got older it retreated to her head and tail tip and then dissapeared complelty, currenlty its returning and she has little blue tufts where her ears meet her skull and has some blue hairs scattered through her coat and at the tip of her tail.
it takes a good 3 yrs before a chihuahua has its adult coat and can change colours till then, especially in long coats. but its fun to look back through the pictures and see the changes.
Yes, the coat can change over time. When our fritzy was a puppy he was all brown, dark at the tips and light sable towards his skin. As he has gotten older the dark brown hair has been replaced with pure black hair and covers the top of his back now. He had no visible black hair until about 8-9 months old.