Hey there Peter! Great question! Here is what I think is tripping up the client: Once their customer pays a deposit on the estimate, it is automatically considered approved. An approved estimate can not be edited, therefore once the deposit is paid, it can't be edited while it is still in the estimate stage.
However, when you convert an estimate to an invoice, you will be given the opportunity to edit it. In addition, an invoice can always be edited, as long as it is not fully paid. 

To sum it up, you're only able to edit a paid deposit after it becomes an invoice. With that said we always want to make sure we're hearing feedback. Is there a certain reason that the client wants to make an edit to an estimate a customer has already paid on (which means, theoretically, they've already approved it) without converting it to an invoice?

Thanks again for your question!