They technically never do the gene replacement within the body. They take the marrow alter it, grow and produce an external modified new marrow. Chemotherapy wipes out the old bone marrow out and the modified marrow is introduced instead.

As breathtakingly advanced and unprecedented as CRISPR treatment is, the execution is still radical and crude by necessity. Not to diminish the accomplishments but to note that we still have a great deal of room to grow.

Hopefully knowledge will eventually advance so that less extreme and unpleasant methods will take its place. But that would be a tough nut to crack.

No replacement involved. The treatment edits and reactivates the fetal version of HBB gene that is inactivated after birth.