What's the difference between hair replacement and hair transplant ?

Depends: If you are referring to surgical procedures those terms are basically synonyms. However, hair replacement is a broad term that could include such things as wigs, etc.

Hair Transplant: Hair transplantation involves using your own hair from a donor site in the back of the scalp and moving them (usually in groups of 1, 2, or 3 hairs) to areas of thinning or lost hair at the hairline and top of the scalp. Filling in with these transplanted hairs looks very natural and can help with both female and male pattern hair loss.

No difference: Both terms represent the same procedure. Hair is replaced with transplantation.