I've used DAY ONE since 2011 and always relied on Dropbox to sync my data across devices and provide a back up. Since I'm paying for iCloud (and didn't want to install the dropbox Mac app) I thought I would change my sync preference over to iCloud.
I thought I would share my experience for anyone that is looking to do the same thing
First off make a backup (just in case), fire up your iOS app and go to SETTINGS --> Export to PDF (this will not be able to be imported but at least makes a nicely formatted journal if the worst happens).
Now fire up Safari and navigate to DROPBOX.COM From here go to the apps/day one folder and save the JOURNAL.DAYONE somewhere, I downloaded and put on my desktop for now.
Once you have your backups it's time to move onto moving over.
On iOS go to SETTINGS --> SYNC and toggle Dropbox to off and then iCloud to on. The sync process now begins, basically it takes whatever you have in app and uploads to iCloud so whatever state your journal is in before the update is what gets uploaded.
Now I had iOS sorted I installed the MAC version and went to PREFERENCES --> SYNC and turn on iCloud sync. Once you have ok'd the message box you should now see the sync happening on your MAC.
That's it, you should now be migrated over to iCloud.