Are ICD codes forward compatible?
9 and 10 have different schema. For example, a displaced transverse fracture of the right femur would be

821.01RT in 9
S72.321 in 10

10 is also more specific. In the example, 9 doesn't tell you that the fracture is transverse and displaced. 

Since ICD10 is basically a better organized and more precise superset of the data represented by ICD9, someone has probably created a mapping. However, I haven't looked into that.