The installation guide for 3.1 includes an entire section on upgrading from 2.x to 3.x which includes using the Import/Export tools to migrate your data, what's not included in the upgrade etc... it can be found here:
"To install and run the Uninstall utility you must be logged in as the account that installed Unity. Access to the directory for removing objects will not be possible if you're logged in as another administrator account."
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.