"What you will also have to abandon is old fashioned ideas like "specification","
Interestingly, what @Mike_137 described was a lot closer to agile than to a document driven waterfall process, in that it clearly valued maximising learning from real experiments, rather than spending a fortune on pointless paper speculations - or 'specifications', as the deluded like to call them.
But that's a digression. On the original point, I never met a GDS person who understood legacy, or even transactional systems. Most of them had a background in Digital media, a fundamentally different thing to a digital business.