How to edit/delete Case Customer relationships?

Hi,

Our client needs a case management system to manage police cases. In Police cases, cases and customers have N:N relationship (i.e.: case can include one or more contacts, contact can be in one or more cases [as witnesses, accused, etc..])

How can I edit the relationship between Case and Customer, so it either:

- has N:N relathionship

- Or, make customer not required, so I can hide it and create a new N:N relationship.

?

Thanks