I recently tried to convert existing text from single-column to 2-column. It produced a single column running down the left side of all the pages. I got round it this way:

Open a new document and format it to 2-columns.
Return to the existing document and select all the text with CTRL-A.
Copy all the text with CTRL-C
Return to the new document and place the cursor at the start.
Right click on the document and you will see Paste with several options. In my case the option on the far right produced exactly the right result, but whatever you choose you will see a preview of it.