The preferred platform today is x86 running either Linux or Windows. Transforming the legacy systems to this platform enables virtualization and even private and public cloud hosting.
Often applications can be re-platformed because they rely on middleware and databases, which function identically on x86 platforms. The enabling factor is to test the application on newer versions of middleware or to get the ISV to support these.
For remaining workloads, code modifications and testing is required.
For workloads coded to legacy operating systems APIs directly, there are code refactoring factories that specializes in transforming source code from legacy platforms to x86.