I don't see any reason why not. I think it was David Wang who said that post Vega they were looking at all memory technologies and would apply whatever worked best. It's clear that compute cards benefit from HBM, but pure gaming cards not so much. Now that Koduri is gone, Lisa Su has a clear long-term plan and the balls to carry it through and AMD aren't quite as skint as they were, I don't see any reason why Navi wasn't designed with a GDDR6 controller from the outset. Hell, AMD might have even designed Navi to work with both GDDR6 and HBM if there's a compute-focussed Navi in the future (although to be honest I doubt that, more likely that's what this "Vega 2" is all about - Navi is a short-term gaming architecture).