There isn't a trigger for this that I've found. (That is, a reactive trigger that will happen whenever the variable changes anywhere. Personally, I want these too, but I can see why they aren't in the software).
I've been able to handle this in two ways:
1) Have two variables instead of one. The old value that only gets set in my checker and a "new value" everywhere else. I can compare to see if it got set Every Tick.
2) Or, I create a function that handles the what I want to do and call it everywhere I change the value.
Neither is truly reactive, but they work.