if you already have 5.1.1 installed , then you can go for system transfer from old ePO to new ePO
follow the link for the procedure McAfee KnowledgeBase - How to transfer/move computers from one ePO server to another
Looks like my migration is not supported anyway because we use Endpoint Encryption.
Looks like I will be doing an in place upgrade instead.
Though you can migrate systems between EPO servers, you can't migrate Users - so it's better to upgrade what you have.