If you simply want to move to version 2303 on the existing Connection Server, you simply upgrade it, but from the question I understand that you want to decommission the current one due to some reason and deploy a new one with all the settings. To following document explains the steps:
https://docs.vmware.com/en/VMware-Horizon-7/7.13/horizon-upgrades/GUID-043C31E0-0E1A-4477-8CA1-1029C...
For the answer to the questions, please see inline.
1. Do we need to apply a separate license (Horizon 8 Standard) for the replica CS?
As noted we plan to remove the current CS, but they are going to be both online for a few days.
------ Not required since you are pointing to the same instance.
2. Do both Horizon 8 Connection Servers need to be on the same version for the replication to take place?
The new CS that we are going to deploy is going to be on version 2303, while the current one is on 2111.1.
------ According to the above doc, you first need to upgrade your current Connection Server from 2111.1 to 2303. Then deploy the new server with version 2303 and point to the old server. By doing so, there is no question of mixing the versions (which is not recommended).
3. Regarding the decommission of the current CS, apart from removing the entry using the vdmadmin do we need to take any further actions before powering of the VM that hosts the current CS?
------ You can just run the command and then shutdown the old server.
4. Can we simply import the same certificate under the new VM's Windows certificate store?
------ I would expect it to work fine.
Make sure you take ADAM backup of the old server and also a VM snapshot in case if you want to roll back.
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