Character type-on changing from lower to upper case

I'm making a computer type-on sequence. I'm trying to get leading character to
initially type on lowercase, but then change to uppercase a couple frames later
(so the whole text ends up as all caps).

I can keyframe this manually with masks and junk. But it seems something doable
with expressions (or text animators).

This might give you some ideas. Create a text layer with lower case characters,
add a slider to the layer, add this expression to the Source Text:

n = Math.round(effect("Slider Control")("Slider"));
value.substr(0,n).toUpperCase() + value.substr(n);

Adjust the slider to covert to upper case.