After you complete the prerequisites, upgrade the FrontEnd pool, publish the new topology and actually run the SfB 2015 installer, the first window that pops up is to check Microsoft for SfB installer updates. Foolish me clicked yes check for updates which in turn pulls down the latest CU that gets installed during the upgrade process instead of installing the RTM and then installing the CU after.
The correct option is to “Don’t check for updates right now”
Error encountered: Error returned while installing OcsCore.msi(Feature_LocalMgmtStore), code 1603. Error Message: A fatal error occurred during installation. For more details please consult log at C:\Users\username\AppData\Local\Temp\2\Add-OcsCore.msi-Feature_LocalMgmtStore-[2018_04_23][21_57_03].log
To fix this I uninstalled what SfB components were installed which was only two. I uninstalled Administrative Tools first, and then Core Components
Next I started the SfB installer again and selected “Don’t check for updates right now”, and was greeted with the original/legacy installer, and not the upgrade window. I clicked Install or Update Skype for Business and went through the normal install process, installed certificates, and went to start the services.
During the startup of the services, the FabricHostSvc and RTCSRV services would not start. I saw events for the Fabric service which were 23041, 53760, 53761. I couldn’t find much online for this issue, but it ended up being the Windows Firewall service was disabled on the Lync Frontend service. I enabled the Windows Firewall service, stopped and started all the SfB services, and then restarting the server and everything came back up. The Windows Fabric service needs the Windows Firewall service enabled, but the firewall can still be off.