It's actually a self service action. Login to the My VMware portal on which the license is registered, go to the licensing section, select Downgrade License from the dropdown menu, select the license, and follow the steps in the wizard. For details see e.g. https://kb.vmware.com/s/article/2006975
sorry, I misread the question. I thought you were asking if a 6.0 license would work for 6.5 not 5.5. As you found out, it won't. You can go to the VMware license portal and downgrade it. But keep in mind that 5.5 goes out of support in Sept on THIS year. You should really be using 6.5 not 5.5.