Are you talking about PL-15 and PL-16? That's different because the weapons bays of J-20s can't be changed unlike the size of UVLS. If UVLS could only fit 2 HHQ-9C then the best choice is to replace it with a more specialized VLS around 45cm x 45cm which wouldn't waste any space and allow the Ships to pack ~100% more SAMs, using different types of VLS cells is quite common and wouldn't be hard for PLAN to implement.PLAAF did, though? Having two almost the same missiles is arguably not the best of plans either.