VDL 2.0
For pure VDL views, i.e. not using a custom runner, the only change that should be necessary is to change the VDL version number in each page.
<vdl version="2.0">
Version 2.0 includes a major rework of page styling so things will look different. But any purely VDL structures and components should remain working.
If you have used any of the previous Bootstrap CSS or element structures then you will need to migrate these to Bootstrap 3.
For any views where you are using a custom runner you need to make sure you upgrade the version number in your VDL source file, as mentioned above. But you also need to upgrade the JavaScript API version number to 4.1 in the HTML file. Take a look at the default VDL runner for version 2.0 to see what needs changing:
<installdir>\insight\lib\javascript\default-vdl-runner-2.0.html