merging separate Notes setups in iCloud
If you enable Notes under iCloud settings on your iPhone you will then have an iCloud section in the Notes app.  In the Notes app you will still have the section 'On My iPhone' with all your previously save notes in there.  You will not lose them be enabling Notes under iCloud settings.  After you enable the Notes feature under iCloud settings you can transfer the 'On My iPhone' notes by going into the Notes app, selecting that directory.  Then select edit at the top right, you will now have to option 'Move All' at the bottom left.  If you select Move All you can then select the iCloud notes directory to merge them with the rest of your iCloud notes.  They will then sync and show up on your iCloud account as well as any other devices using iCloud notes, including your Mac.