Just to confirm...

I recently had a DONE3 ticketed (by CX) for departure ex HND in mid October.
I may need to change a stop over (-SFO-xORD-ATL-ORD- to -SFO-xDFW-RDU-ORD-).

Would this trigger a reprice if I did it before departing Tokyo, or just the $125USD fee either before or after departure?

The only time I've done a routing change on an xONEx was on the back of government travel warnings and airline wavers (BKK riots a few years ago), so didn't cost anything.