What controls are you talking about exactly? Even deprecated it should still be working unless highly customized. Having said that moving from 7.2 to 8.4 I'm not sure a like for like approach will be the best one. You definitely wants to work with our new features for a better user experience.
Update Controls effectively render your application user interface in HTML 5 document type (standards mode). Rendering your application in HTML5 document type ensures cross-browser and cross-platform compatibility and use of the latest HTML markup, among other benefits.
You can update a number of controls automatically. Other controls may require a manual update.