Can't really say ....
Other than CZ claims they are not interchangeable .... and, they want you to buy both
My guess is; the length of the Dust Covers .... the P09 KADET Dust Cover,
MAY NOT reach to fit into the Dust Cover on the P07 Lower
Will a (9mm) P09 Upper run on the P07 Lower ?
The OAL of the P09 is what, about and inch longer than the P07 ?
The difference in the Barrels are about 0.8"