Whether documents are versionable is enabled and disabled at the document class, not on individual documents. After you create a document that uses a class with versioning disabled, that document is permanently non-versionable, it can never be checked out, and it cannot be converted into a document with versioning enabled. There is always only one document version in its version series.
Content Engine object model rules apply to documents with versioning disabled. You can change any modifiable custom properties that were assigned to the document class. An example might be a description field; you can change the value of this description field, but this would not create a version of the document. The effect of such a change, of course, is that no record of the change is maintained anywhere in the document properties or version history.
If a document with versioning disabled is a minor version, you can promote it to a major version. Conversely, if it is a major version, you can demote it to a minor version. See Versioning actions.
Changing the versioning-enabled setting for a class (in either direction) does not affect existing document instances. In other words, the versionability of a document series is set at the time the initial version is created and never changes. To see this behavior, set up a document class, make sure its Supports Versioning check box is selected, and add at least one document based on that class. Then clear the Supports Versioning check box on the document class property sheet and add a different new document assigned to the class. Then try to check out that second document, and you will see that the object store prevents the checkout, whereas the other document that was added before you turned off versioning support can still be checked out.
Document versioning is set on the document class, using the General tab. The setting on the Support Versioning check box can be changed. The versioning setting on specific documents is inherited from the class. The value of Versioning is read-only and cannot be changed: