re-IPing a Public IP network to Private IP

Happy Hump Day,

I've been tasked with moving a 30,000 user network from a public IP'd network to a private IP'd network. I've done some searching but would like to get ideas or a basic plan of attack for this project.

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Background of the current network state:

- Flat Layer 2 design from access to core

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Unfortunately, this is in the beginning stages of the project and all the information I have on the network. Any ideas, things to watch for, obvious no no's would be much appreciated.

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Thanks in advanced!