I have done a couple of these upgrades. There are a couple of ways you can do this:
Create new Enterprise server and new Deployment server hardware/software/database configuration on separate network.
Backup old system and restore to new
Disconnect old system and connect new
Go live with new configuration.
Create new Enterprise server hardware/software/database configuration and add it to existing configuration.
Move DEV and CRP environment to new Enterprise server configuration.
Install XE on new Enterprise server and convert DEV and CRP to XE
Upgrade PROD to XE and move production database to new to new configuration.
Remove old enterprise server
Upgrade SQL on old server and use as Development server or logic server.
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