Upgrading PSC 6.0 appliance to 6.5 worked like a charm.
Next step was vCenter appliance upgrade,which would now consolidate Windows based VUM server to the vCenter appliance. So this requires you to run VMware Migration Assistant on the Windows VUM server.
Every time I start the Migration Assistant it failed with an below error
Error: A problem occurred during authentication to the legacy vCenter Server using the provided credentials. Resolution make sure the vCenter Server is up and running. Verify you have entered the correct credentials.
I was sure that the SSO password is correct.
Looking at the logs
UpgradeRunner log mentions
2018-06-20T21:48:04.483Z INFO utils.featureState_utils Found FSS AUTOMATED_VUM_UPGRADE=True
2018-06-20T21:48:04.513Z WARNING bindings.vc_binding Cannot get access to VC Inventory http://<VCIP>:80, because of Error: Server has wrong SHA1 thumbprint: 7aa2612f090cdd1212sqw216943489ada70019 (required) != b3de7868330f12511b767a122abb245448d5d (server).
2018-06-20T21:48:04.515Z ERROR UpgradeRunner Cannot log in to legacy vCenter Server
2018-06-20T21:48:04.516Z INFO output.requirements_result_producer Repository is not set, not adding SSO information.
2018-06-20T21:48:04.529Z INFO output.requirements_result_producer Persisting preupgrade result :
FIX
1) Navigate to C:\Program Files (x86)\VMware\Infrastructure\Update Manager
2) Open "VMwareUpdateManagerUtility"
3) Provide vCenter FQDN or IP and Login with "administrator@vsphere.local" & SSO Password
4) Under the left side options list, click on "Re-register to vCenter server" and provide SSO credentials again (administrator@vsphere.local and password)
5) Once done, restart "VMware vSphere Update Manager Service"
6) Try Migration Assistant again
It's happy this time!!