SAP HA solution for existing environments

Hi,

We have an existing running ECC 6.0 system with ABAP stack(NUC), which we recently upgraded. We are in the design phase for making this system Highly available (HA). I went through the installation guide, which talks about HA. The guide refers to HA configuration from fresh installation perspective. Has anyone does this before or has any idea/documents about how to make an existing environment HA.

Thanks for your help.

Jitendra