I can't really help you with xRegistry editing, and anyway I think that is the more complicated option. The second method is pretty straight forward:
1) Backup your saves from your previous account (copy them to USB through XMB, unlock any that are copy protected using Apollo).
2) Backup your licenses (you can use Apollo Save Tool for this, export your .rifs as .raps to a USB).
3) (Maybe) backup your trophies (again, using Apollo, but if you synced all your trophies you don't need to do this).
4) Create new user through XMB
5) Log in to PSN, and activate your account
6) Copy your save files from USB to your new profile (again, though XMB with save data management)
7) Restore your licenses with Apollo (export your .rap as .rif files)
8) Restore trophies with Apollo (if you did create a backup of them).
9) Delete your old user if you wish.
All your save files should be playable, but if not try resigning them with Apollo. You should be able to figure out how to use Apollo for different things on your own (the UI is pretty straight forward).
Edit: If someone knows an easier way to edit the accountID without copying the xRegisty to PC and manually editing, please do share.