If you want to install Invoice Manager 1.4.0 over an installation of Invoice Manager 1.3, you need to consider a couple things.
We have simplified the Invoice template system in order to be easier to customize it. This means that when you install the 1.4.0 version, you will have 2 new templates alogside your old ones. One for the Invoices and another one for the Quotes.
The invoices that you have created with the old template will continue working. Even the new invoices with the old templates will work. However it is highly recommended to update the fields Default invoice template and Default quote template in the general configuration of the component to start using the new template sysyem. This will provide you new tokens and functionalities you can include to your templates including Grouped taxes by items and invoices in different languages.
Note: When you change the old templates for the new ones remember to replace at least the JoomlaThat! logo with yours.