Thank you for the reply!
I know that your interest is that I'll buy the module , but would the SQL query:
UPDATE ps_customer SET id_default_group = '3' WHERE is_guest =0;
be correct, if the aim is to get all the customer accounts, who are not guests, into the Customer Group 3?
Edit: Or there should probably be another field, as manually I do change two fields: Group Access and Default customer group...