Hi,
I will suggest you to carry out a POC for few devices before you rollout to the wider audience and also follow the detailed guidelines from MS, basically the SCP configuration is a pre-requisites to setup hybrid deployment and this is known as targeted deployments where you deploy only on few devices.
For Azure AD Connect the process is automated with Azure Hybrid - https://learn.microsoft.com/en-us/azure/active-directory/devices/howto-hybrid-azure-ad-join#configure-hybrid-azure-ad-join
Targeted Deployment on few devices follow this - https://learn.microsoft.com/en-us/azure/active-directory/devices/hybrid-azuread-join-control
There is no impact on the already joined PCs and make sure you have backup of all the configuration and documented so it helps you to plan out accordingly.
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