You can sign in using your old email address and the password you used for uploading pictures.
If you never signed in before, you'll need to first click on the confirm link in one of your previous emails from Pixie. If you don't have one of those, you have no way to sign in. In that case you should reupload your galleries (which show up as pix01 playlists) with the new email address/password.
There probably should be a way to change the email address on an existing, confirmed account. If you are not confirmed though, that wouldn't help you.