From my experience, 95% of local variables "just work" after turning null safety on. Possibly more. But to be clear, this is coming from only migrating 2 small-ish packages.

For example, presumably you're not doing:

var name;
if (isCzech) {
  name = "Filip";
} else {
  name = "Philip";
}

Because in that case, yes, you’ll have to put String there instead of var. But things like var buf = StringBuffer() are fine, unless you expect to put a null there later.

There's also going to be a migration tool that will do the tedious work for you.