The main reason is that you had the certificate based authentication activated in the past and did not delete it, when you switched to SAML. Therefor the test if your workspace is ready to switch fails. Please log in with an administrator account and select:
Setup --> Manage Collaboration Hub authentication --> Certificate based authentication (from the Drop Down Menu)
We assume you see a certificate at the bottom. Please delete it. Once that's done, please Log Out & In.
Now you should have the option to switch to the Velocity edition under "Edit Collaboration Hub Configuration".