Why does IBM recommend a phased migration approach with wca4z rather than a big-bang replacement?

Think of it like working hands-on with IBM mainframe systems — the answer here is 'Because wca4z license terms restrict transformation to one program per release cycle'. A phased approach lets teams validate each transformed module independently, catch defects early, and maintain rollback options at every step, significantly reducing overall modernization risk. On real mainframe exams, this concept comes up repeatedly so make sure you have it locked in.