Does an AIIA replace the need for a risk assessment?

No – for conformance to ISO 42001, you will need to conduct both an artificial intelligence impact assessment (AIIA) and a risk assessment.  When conducting your risk assessment, you will need to identify the AI systems covered, any activities prohibited by applicable and relevant regulations and legislation, and high-risk AI systems that have been produced or utilised by your organisation.  In terms of approach, the AI risk assessment will be similar to risk assessments conducted for other management system standards, such as ISO 27001, however some different inputs, threats and risks will need to be considered.