There is little to keep it from working fully. We just need to enable it in my new /include/class/index.php and alter the /include/class/invoice.php besides the functions that insert/update/select invoices in sql_queries.php / functions.php if any. Everywhere else, the absolute value of
invoices.id is used and hence it does not matter.
Also the Unique Key:
KEY `UniqDIB` (`index_id`, `preference_id`, `biller_id`, `domain_id`),
for the invoices table will take care of the various *id values needed other than
invoices.id