I just triedFor clarification, my issue is that I do not use either of the email accounts I had when I signed up my two accounts with Horizons back at launch and need to have them converted to one I now use.
https://accounts.istaria.com/member.php
And it does not ask me my email at all, just the character name.
I just tried
https://horizons.istaria.com
and it asks me for the user name (optionally the email) and then it works perfectly.
What do you need the email for?