Martin Fowler reposted a good blog post on Strangler Applications a couple of days ago: Strangler Application.
This is a practice of replacing legacy platforms peice by peice rather than all at once in a rewrite. His discussion centers around using this method to in order to mitigate risk, which is of course a massive advantage.
What he doesn’t focus on is how this also fits far better with Agile delivery methods, keeping stakeholders engaged as each section is replaced with the new version, allowing the business to refocus, prioritise and adapt as we collect feedback on the stuff going live, all core to an agile approach.
There’s also a link to a summary of case studies where people have used this approach. Both are worth a read.