New physical design, fresh masks, and a re-do of validation. Creating a derivative means re-using much of the R&D that went into the base architecture which the prior GPU did from its base architecture, so if that cost is discounted, it is the costs of implementing, validating, and manufacturing a new ASIC, plus possible costs related to inventory of the prior GPU that does almost the same thing.Obviously, I meant how feasible is that they'd just update the GPU with newest tech and leave it as it is otherwise
That AMD has been reluctant to do so the last change it got may indicate that the return on investment is not great.