What I meant was to remove the web services on the printer itself, then re-enable them. Maybe that will reset the email address such that it can be registered. From the HP support page:
"On most printers, you can use the Remove Web Services option to disable all Web Services. This will remove the printer from your ePrintCenter account, delete all print apps settings and configuration data, and also delete your printer's ePrint email address/account and configuration/setup data. For Photosmart printers, the Remove Web Services option is available either under the Setup>Wireless>Web Services menu option, or the Setup>Web option, or via the ePrint icon on the printer front panel.
If you later decide you want to re-enable Web Services to use print apps or ePrint on that printer, use the Enable Web Services option from those same menu locations."