How can I change the receivable Sales Invoice Numbers?

I have to enter different types of revenue as invoices, and use unique numbering for different department invoices. In the settings the I am unable to disable the "Next Invoice Number" setting. 

Nor in the "create new sales invoice" - I cannot add an invoice number.

Nor "edit invoice" function - I cannot change the invoice number. 

Resulting in many voided sequential invoice numbers, as I can not create/replace a new invoice with the same number already used. 

How can I disable the Next Invoice Number setting or at least overwrite it if the invoice number is incorrect?! 

And, as far as I can tell, there is no easy way to edit an invoice, including the # after it has been paid. Which is very cumbersome in terms or having to unallocate the payment, edit the invoice and reapply payment. If the transaction is already reconciled with the bank, it seems even more of a headache. 

Here's a screenshot of my settings page: