Then you delete the invoice and redo all the work with the method you wish you had used, first. But since the invoice exists already, you need to recreate it so that it doesn't change the financial reporting; you recreate it to be the same as what it was. And, since there was no estimate before, there is no need to create an estimate to invoice from, now. All of this is just busy work, really. The existing invoice wasn't created from an Estimate and doesn't have to be created from the estimate, to give good reporting. If you were not going to create from the estimate, anyway, then it doesn't matter which sequence they get created in, now.

In other words, without knowing more details for what needs to change, just making an estimate now is no different than having it there previously.

We really are answering everything about how all of this works, and not any specific reason to do anything specific at all.