Hi,
Well, modification of HikaShop core files is not recommended because you won't be able to update HikaShop without loosing your modification.
That's why I was talking about a little custom plugin.
In the plugin, with the trigger I gave you in my previous post. You can simply load the HikaShop user details and replace the user_email with the "old" user_email.
From that point, you will be able to improve the plugin ; make some tests on the "new" user_email, display a warning/error message, etc.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.