Will new workflows be applicable for existing transactions?

No, it will not work for existing transactions.

For example, let us consider the example wherein an invoice was created on 1st December 2014. You create a workflow rule on 3rd December 2014 to send out email alerts to you and the customer 10 days after the Invoice Date i.e 10th December 2014. In this case the workflow rule will not be applicable because the workflow rule was created 2 days after creating the said invoice.

However, you can clone an existing invoice after creating the workflow rule for the feature to work.