Workspace Admins cannot update user field types but they can update other field types.
View from the Subscription Tab
Release : SAAS
This is working as designed. Most of our fields are created at the Workspace level and are workspace scoped, so Workspace Administrators can edit those fields.
The User Fields are created at the Subscription level because these fields are used on all workspaces across the Subscription. Because these fields are used for all workspaces, only Subscription Administrators can edit them.