MOVING iCAL CALENDARS BETWEEN LOCAL AND iCLOUD

On joining iCloud your calendars will be moved to iCloud, including any 'On My Mac' calendars. You may create, or still have, calendars listed under 'On My Mac'. These latter cannot sync with iCloud or be shared with other users but can be published to a web page. The iCloud calendars are actually on the server and so are visible to other devices and can be shared (publicly or privately), but cannot publish to a web page. (Local calendars can be published to a web page if you can find a CalDAV server to do it - an ordinary server won't work.)

If for any reason you wish to move an 'On My Mac' calendar or calendars to iCloud, or vice versa:

To move 'On My Mac' calendars to iCloud

1. Select your 'On My Mac' calendar. From the File menu choose 'Export...' then 'Export...'; from the sub-menu. This will create an .ics file at the location chosen in the Export dialog.
2. From the File menu choose 'New Calendar' then your iCloud login name in the sub menu.
3. A new calendar called 'Untitled' will appear in your sidebar under your iCloud login name. Change its name to that of the calendar you exported.
4. From the File menu choose 'Import...' then 'Import...' from the sub-menu. A navigation pane will open: navigate to and choose your exported .ics file.
5. iCal will ask you where to import this to. Click the drop-down menu and select the iCloud version of your calendar (make sure not to choose the 'On My Mac' version). Import.
6. Your calendar contents will appear in the iCloud version of the calendar but this may take some time. You need to leave iCal running until the rotating icon at the top of the sidebar disappears.
7. You can now delete the 'On My Mac' version of the calendar (control-click it in the sidebar list and choose 'Delete').
8. Repeat for other calendars.
   

To move iCloud calendars to 'On My Mac'

1. Select a calendar listed under your iCloud login name.
2. From the File menu choose Export...>Export. An .ics file will be saved at your designated destination.
3. Create a new calendar with the same name as the one you've just exported from, choosing 'On My Mac': you may need to sign out of iCloud first to do this.
   
4. From the File menu choose Import... Select the .ics file and choose to import it into the calendar you just created (make sure you don't import it into the iCloud version).
5. When complete, check that it's OK and then you can delete the iCloud version.
6. Repeat for other calendars.

Performing this process may lose your calendar colours which you will have to set up again.

In both cases I should retain the exported .ics file(s) until you are sure everything is working properly.