The usage here can get pretty complicated. If I change clothes, that simply means that I take one set of clothes off and put on another. If I replace a shirt, however, that would imply that I don't have the original shirt - that the original was thrown away. If I change the spark plugs on my car, however, I have also replaced them - I have thrown out (or recycled) the old spark plugs. If I am dancing, I can change partners. Technically I am replacing a dance partner, but you would never hear it that way. The way that "change" and "replace" are used in English depends as much on custom as to any strong logic or rules.

To my understanding, CHANGE means one thing become another after a process. REPLACE means the it is not the initial thing at all. A totally new thing come to the position of the original.