The "Convert to..." function creates a new document.  It does not append changes to an existing layout.

Stardraw is an MDI application (Multiple Document Interface) which means that it can open many documents at the same time.  You can switch between these documents using the Window menu.  I suspect that you are unable to save the newly-created document with the old filename because the old file is open, and therefore protected.

Recommended use would be to complete the work you need to do in the Block Schematic environment and then, when finished, Convert to Rack.

I hope this is helpful.