The redesigning approach is most complex but provides maximum benefit. You can choose this approach if the legacy system is completely outdated and is not able to accommodate business needs at all. With redesigning, you need to build the entire system from scratch while keeping the overall scope intact. The following diagram shows the legacy mainframe system migration to AWS Cloud:

Here, a legacy mainframe system is rearchitected and refactored to similar cloud services as a modernization approach. Building a cloud-native application helps you utilize and fully benefit from cloud services in aspects of scalability, performance, reliability, and cost. It helps your team to be more agile and innovative by accommodating rapidly changing technology in your system.

Redesigning a legacy system requires a long-term project with lots of effort and increased cost. Before kicking off a large modernization effort, as a solution architect, you should do careful analysis if any Software-as-a-Service (SaaS) product or commercially available off-the-shelf (COTS) products are able to handle your business need with relatively lower cost. It is essential to do a CBA for redesign versus purchase before proceeding with the redesigning option.

Sometimes, it's more beneficial to replace the existing legacy system with new third-party software. For example, your organization may have a decade-old Customer Relationship Management (CRM) system that is not able to scale and provide the desired feature. You may look for the option to subscribe to SaaS products such as Salesforce CRM to replace the legacy system. SaaS products are subscription-based and offer per-user licenses, so they may be the right choice if you have a smaller number of users. For a vast enterprise with thousands of users, it may be more cost-effective to build its application. You should conduct a CBA to understand ROI when investing in SaaS products.