As the name implies, RSForm!Pro Legacy Layouts plugin will allow you to bring back the legacy layouts in your form.
When the installation process is complete, you will have to navigate to Extensions > Plugins and make sure that the "System - RSForm! Pro Legacy Layouts" plugin is enabled.
Afterwards, you will be able to head to Components > RSForm!Pro > Manage Forms > Select your form > Form Properties > Form Layout and select one of the Legacy Layouts.