Unfortunately, single usage of "replace_block" does not allow you to replace a whole subsystem.
The "replace_block" is used to replace the simulink block, not the entire subsystem, so the "replace_block" API alone cannot be used to replace the entire subsystem. To implement exactly the workflow you want, you need to use the "add_block" and "add_line" functions to copy and paste all the contents of the source subsystem and associated port configuration into the target model.
