Existing account with working sync; password lost

I have a working Firefox sync account, running on two computers already. I tried to log in on a new computer, and received a "Password Incorrect" error. I think this is related to the change to Sync in Sep 2015. My account was created in 2012, and the password is stored in a password manager, so I know it is not lost and not incorrect. The password was last changed in Aug 2012. I tried following the instructions here: https://support.mozilla.org/en-US/kb/sync-your-firefox-bookmarks-history-passwords-and- but there is already a Sync account with my email address, so it doesn't work. How do I create a new Firefox account with the existing email address, without losing the existing information? Is it best to delete the account with a connected computer, then create a new account and synchronise again?