I would like to copy a previous Canvas course into another existing shell. When I course copy by instructions, it creates a new shell altogether.
This is something that our faculty do hundreds of times each semester. Are you following the instructions at:
How do I copy content from another Canvas course using the Course Import tool?
What you need to do is go to the new course where you want to put the content, then "import" from the previous course:
What is the Course Import Tool?
Retrieving data ...