Have you tried signing up with this URL? : k12.instructure.com ?
You can only access K12 edition if you select K12 during sign up, so try using the URL instead of the main page.
Something you can do is share your course to commons. Commons is an object repository that allows educators from around the world to share resources with each other. So, on your canvas.instructure.com edition, go into your course settings, then on the right select share to commons. Keep in mind that with the canvas.instructure edition, you can only share resources with the public, not just yourself. So if you do not want other people to see this course, I recommend sharing it when you're ready to copy it.
Once you have successfully shared your course to commons, open your k12 canvas account, open commons and import that course you shared into a k12 course (the course must be already created prior!). Once you select import the import process will begin, and your course will appear! From there, if you still don't want the course available on commons, you can return to your canvas.instructure account and remove the resource from commons.
Let me know if this helps you out, and as always, respond with any questions!!
Noah