Change guest registration to registrated

Hi,

There is no such error message in HikaShop.
The only error message similar to what you're talking about is the other way around:
If someone registered with an email address then try to do a guest checkout with that same email address, it will display an error message saying that there is already a user account with that email address with a link to reset his password if he forgot it.
So in such case, the customer just has to click on the link to reset his password and login.
Note that in old versions of HikaShop, there was no such link, only the first part of the error message.
And it's possible that you didn't get the new part if you have some view override or translation override (or didn't update your HikaShop translation file). So in your case, I would focus on this so that the customer can easily reset his password if he doesn't know it anymore.