Question: How to convert Attachments owned by "Self Service Users" to ContentVersions?

@Brian Roff was using my Attachments to Files conversion tool and he encountered error "FIELD_INTEGRITY_EXCEPTION, Owner ID: id value of incorrect type: 035xxxxxxxxxxxxxxx: [OwnerId]" (https://twitter.com/BMRideas/status/848955224345759747)

I learned the id prefix 035 means "self service user" (https://help.salesforce.com/articleView?id=Standard-Field-Record-ID-Prefix-Decoder).

According to https://help.salesforce.com/articleView?id=customize_selfservice.htm "self service" isn't available to orgs after Spring '12, and I don't have any orgs that old so I don't have access to create these user license types.

Is the answer as simply as on the self-service user record check the box "Salesforce CRM Content User"?

Thanks!