I have been deploying a free 60 days vSphere Essential Kit Plus optimized to be used on HPE DL380G10 Server from vmware website.
Now I have purchased the HPE vSphere Essential Kit plus from HPE.
shall I download a new fresh copy of package & install it on the server & activate the license, or the present & running copy of the package is sufficient for activation?
You can install your license keys without reinstalling vcenter and ESXi hosts.
As already stated above, you don't need to reinstall, assuming your license is to same major version. For instance, if you installed vSphere 6.5, you will need a license for vSphere 6.x... no matter if the ISO comes from VMware or if is the customized HPE ISO.