No, nothing is replacing the other.
921 is the further development of the concept "CZ-5DY", a crew-only carrier rocket. CZ-9 is the cargo counter part in the moon mission.
From day one, China's moon mission consists a crew carrier and a cargo carrier.
The only publication I have seen so far (2019) says that China's moon program will begin with surface survey-> surface preparation -> tooling delivery -> basic surface habitat building -> human landing ->.... In this plan, CZ-9 would be used for material landing before 921 put man on the moon.
This plan is not necessarily the only path that China is thinking of. But if China is not under the pressure of competing with US, it is probably the preferred path. The initial targets of both countries were around 2030.
Some people has suggested that, because Trump moved US' manned landing from 2028ish to 2024, earlier 921 landing a man on the moon is preferable. It is essentially a move for bragging right, flag raising act without materialistic benefit. This is only some armatures' wish, not backed by any publication.
So, if I can get this straight...
The 921 rocket is essentially a "CZ-5 Heavy", an offshoot project meant to fast-track CLEP's manned lunar missions, and in many ways comparable to the crewed SLS Block 1. Its design consisting of 3 CZ-5 cores heavily suggests that it's meant to use as much off-the-shelf technology as possible. We can expect this launch vehicle to debut sooner than 2030 which is when the CZ-9 is expected to fly, consistent with photos that the core structures are already in testing.
The CZ-9 is more of a long-term venture designed to haul heavy cargo to lunar orbit and beyond and will not be man-rated. I expect this project to be more on the backburner, with the 921 project stealing the spotlight (and with that most of their engineering talent). In other words, it's the CNSA equivalent of the SLS Block 2 and will likely fly in the same time frame (or maybe later since delays seem to be inevitable with the Chinese space program).
Sounds about right?
So far the Chinese seem to be emulating the SLS program down to a tee.