Hi,
I’ve made a post here: New unified variables and variables declaration in GDevelop 5.4 if you’ve not seen it already 
I would need to know whether I need to manually adjust all events or not.
Remember that no, you don’t need to manually adjust all events. They continue to just work. When you work with a variable, remember to declare it otherwise the new action/condition related to the variable won’t work and will complain that you must declare it.
But that’s it. Are you making a new action/condition using a variable? Declare it. Change nothing else.
To help you, GDevelop displays undeclared variables in the variables editor (but can’t automatically declare them because it has no sure information about the type, initial value, etc…). But again, you can do this progressively as you work on your game (or take a bit of time to declare everything).
The new variable system won’t change, though we may add some links to quickly declare a variable in some place in the UI.