Hello,
Do you want to upgrade the existing BE 8.6 installation to a Backup Exec 9.1 ? Or do you want to just install a Remote Agent 9.1 on the server? In the second case you can directly install the Remote Agent 9.1 on the server which has BE 8.6. You don't need to uninstall BE 8.6.
Please refer to the following document :
http://support.veritas.com/docs/253058
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Thanks.