I don't think this is rocket science -- they're just looking to you to demonstrate that you have the skills to do the job. It sounds like you're applying as an experienced hire -- so presumably you know what a transformation plan is and what it should contain, right?
It's kind of like the watercooler-pitch test. If a client came up to you and said "My business is facing a transformation challenge (e.g. implementing SAP and adopting the SAP business processes) -- what do I need to do over the next 18 months to ensure a successful transformation?" You've got 10 minutes to provide the headline view.
The interviewer will be looking for a few things:
- Awareness of the full project lifecycle
- Pre-transformation: project planning, org assessment, capacity for change, etc
- Mid-transformation: project management, delivery, change management, benefits tracking
- Post-transformation: benefits realisation, performance measurement, embedding the change
- Awareness of the softer side of consulting: roles and responsibilities, attitudes, "power maps", influencing and negotiating skills, getting buy-in, etc
The list above is not exhaustive, but should give you something to begin thinking about.