The Sega Mega Drive did something similar, normally the Z80 is just a support CPU used to run sound (although the 68000 could also talk to the sound chips, so some games run sound off the main CPU to varying degrees). But with the Master System converter, it's the main CPU for backwards compatibility. The converter itself is largely passive to adjust for the cartridge connection differences and adding the pause button, all the backwards compatibility is handled by the main system.

On the PS2 side, apparently later systems replace the original IO chip with a PowerPC based chip running a MIPS emulator, which is kinda wild in itself.