Large legacy systems feel scary, but you got this. Let’s

• Use an ESB or message broker to hook one legacy piece at a time. Expose old JSP pages or procedures through new APIs.

• Put a gateway in front of the legacy apps. Gradually route calls through it and keep the old plumbing under the hood.

• Pick a simple function (say user lookup) and reimplement it as a microservice. Let the legacy system handle requests until your new code is stable.

• Define a simple shared user/account data model as you go. Use the ESB or gateway to translate old formats into the new model.

• Replace features one by one. Don’t bite off everything at once. Keep the old running until the new piece works flawlessly.

• Write tests, monitor traffic, and deploy carefully. Each little victory is progress.