Hi Marcus,

You can follow the following steps. I tried it just now and it still works. The key point is you can use your “real” kindle device to do this trick. Kindle for PC or Kindle for Mac works as well.

a. go to http://www.amazon.com
b. login to you account
c. Manage your kindle
d. Manage Your Devices link in the left panel
e. Turn off Whispersync Device Synchronization
f. Open the book from Kindle for Mac
g. Go to the new location and close this book
h. Click the Refresh button
i. Re-enter the book in the Kindle for Mac and click the Refresh button to make sure the new location works
j. Turn on Whispersync in you Amazon