This is already happening, by the way. SK Hynix's new Cheongju M15x fab was originally intended to be for DDR, but mid-construction they shifted it to HBM. Sim

This is already happening, by the way. SK Hynix's new Cheongju M15x fab was originally intended to be for DDR, but mid-construction they shifted it to HBM. Similarly, the Icheon M10F packaging line has been retooled to HBM, allowing it to assist in processing the output of other fabs. And this was news early last year, so who knows how far it has progressed since then.