Switching CPU architectures is not a difficult process for a modern user application. Applications are written to an operating system ecosystem, like Windows or MacOS, not to a CPU architecture. Before a new CPU architecture can be introduced, the operating system has to be first converted to use the new architecture.
A great deal of work is first done by the OS vendors (Microsoft or Apple) to prepare for a new CPU. Once that work is done can end user software developers port their software to the new CPU platform. Windows has run on several CPU architectures - Intel X86, ARM, and Itanium. MacOS previously ran on PowerPC chips and now runs on Intel X86.
When Mac Rhino was first released as a WIP, it ran on PowerPC Apple computers. When Apple later made the transition to Intel X86 CPUs, Mac applications contained two versions of the application code - one that ran on PowerPC chips and the other that ran on Intel chips. When the application was started, the OS picked the application version appropriate for the host computer.
So the core Rhino code has already been through CPU architecture changes. All of Rhino’s code is written in high level languages that are translated into CPU specific instruction by operating system tools, so there is very little in Rhino that is dependent on the CPU. There was a little work necessary to adapt to the PowerPC architecture for the first Mac version, but this was very small compared to the effort that Microsoft or Apple must do to create a stable and robust OS platform.