Since you're using an external MSSQL database then the upgrade will not change that.

If you are using MS SQL Express on the vCenter Server then yes, it will be converted to vPostgres (see vSphere 6.0 Documentation Center). However, in vSphere 6.0 the vPostgres database that is part of the vCenter Server Appliance, delivers the same scale as MSSQL or Oracle. There are still some scale limitations with vPostgres on Windows (see https://www.vmware.com/pdf/vsphere6/r60/vsphere-60-configuration-maximums.pdf).