Ok, just tried it out: so the move and width agree and the

i

still works--i.e. any scripts/code that doesn't expect snap internal will continue to work (right?)

Yes, that's right. The whole setup is a sort of uneasy compromise between magic's original scalable (lambda) units and the fact that scalability became untenable some time in the late 1980s or early 1990s. Lambda units are still the fixed reference dimension, and the base "internal" units are scalable.