WTDocument checkout / in is fundamentally different from CAD Document checkout / in.
CAD creates relationships between files (e.g.assy <> component and drawing <> component or assy). For this reason, filenames need to be constant.
For WTDocuments (non-CAD), think of the "document" as a container for a file (actually one or more files if attachments are used). The contents of the container are replaced with each checkin. You can in fact create a WTDocument with a Word file, then on check out, replace it with an Excel file.