In those cases, the switched-to architecture could emulate the previous one with acceptable performance. For the hypothetical amd64 -> aarch64 transition, I'm unaware of the existence of software, silicon or the combination of the two which could emulate the former on the latter while staying in a reasonable power envelope.
Emulation is non-optional if one wants to avoid splitting the ecosystem.
Emulation is non-optional if one wants to avoid splitting the ecosystem.