overwrite version of file?

In most cases, I agree that versioning files is the optimal way to go.  But I'm confused as to when it's "permitted", "not permitted" and "mandatory".

For example, I can't find an API in the docs to overwrite - or "save as same version" - of an existing file (say after editing locally).  However, it is my understanding that the Free/Personal Box accounts do not allow versioning of files at all?  Then how would you upload an edit to an existing file for a personal account?  So for personal accounts, versioning appears to be "not permitted", but there's no API that lets us update a file's content.

For enterprise accounts, versioning is enabled, but is it required?

Can someone clarify file versioning and any additional APIs to facilitate uploading new content to existing files (other than https://docs.box.com/reference#upload-a-new-version-of-a-file )?

Thanks,

AJ