According to Tweakers, a spokesperson for Sony told them that existing Sony cameras will not able to get Sony a7S III(B&H/Adorama/Amazon)’s new menu. Sony spokeperson said such an upgrade is ‘technically not possible’.
From Tweakers:
Sony reports to Tweakers that this will not happen and claims that this is technically not possible because the A7S III has a new design with a new chipset. However, the spokesperson expects that future cameras will receive the new interface.