If they'll ever do it, they must be careful to avoid splitting the audience, back and forward compatibility between 4.0 and 4.1 games must be kept at all costs, and not only this, as marketing must also be absolutely clear and avoid at all costs to confuse users about it. Such a move could make sense if they plan to keep PS4 on the market even longer than PS3 and possibly than PS2, in this case they could arrive to a point when upgrading APU and RAM could cost less than keep on using the original and by then legacy (although probably die-shrunk and refined) ones. Also, such upgrade would most surely happen only when a sensible boost could happen without increasing power consumption. I wonder, though, whether such move could make things more difficult for PS5, raising the expectations for it, or if, keeping on using an AMD APU architecture even for 9th gen, it could allow a smoother transition, with full BC to help PS5 in the first year after it will run out of early adopters.