I am having the exact same issues. This is really ludicrous that we can not add an existing invoice to a project. To delete an invoice and then redo and change the invoice is illegal in our accounting laws, this is really not an option.
Its a basic functionality that is required of using a projects function..... surely this is now years on and this has still not been resolved.
We do hundreds of quotesd and can not possible create a project for every singel quote... seriously .... ...
