What are the most promising ways that GenAI can help application modernization efforts? Have you had any success using GenAI for this?

I'd combine the responses of Didier and James, create and/or improve documentation of legacy applications especially those where SME's have or are about to retire, then refactor/optimize as a step towards modernization. The last step, if that's on the roadmap, is to transform where you may want to take your legacy application and transform it into a more 'modern' platform. 

Code Refactoring and Modernization

GenAI helps us reverse engineer undocumented legacy systems and lower the cost and risk of replacing them.

I've seen one too many vendors try to pitch AI application modernization tools.  I would first start with an application rationalization exercise using the Gartner TIME methodology.  Once you identify the Invest/Mitigate/Eliminate applications, you then can look at tools to modernize the applications.

Application support via ChatGPT has been great for me. When I want to accomplish something via any commercial application and I ask ChatGPT and it’s way better than almost every applications built in HELP or trying to Google tips.

We are looking into it for our in house applications but that’s going to take more work than we first thought