if you have a back-up of your old .encfs6.xml file,
then you need to replace your new .encfs6.xml with the old one and try to mount with the old password.
The .encfs6.xml file is located in your Boxcryptor (.bc) folder.
Please make a backup of your data (.bc folder) before doing this. If it won't work, you'll have to try to remember your new password.