I would prefer not to do re-implementation of a legacy system as a project. For a start legacy systems often have a lot of dead features that nobody uses any mo

I would prefer not to do re-implementation of a legacy system as a project. For a start legacy systems often have a lot of dead features that nobody uses any more, so we risk wasting time implementing things nobody needs. A 6 month backlog also sometimes means a build-it-all and release approach which means 6 months before we start using the replacement in production. I prefer to take a strangler application approach http://martinfowler.com/bliki/StranglerApplication.html http://martinfowler.com/bliki/StranglerApplication.html Start replacing small parts of the legacy system in production as rapidly as possible. Then move across bits of functionality bit by bit until enough functionality has been moved to turn the old system off. This also allows you to rapidly get benefit from new functionality that the old system did not have, or improved performance for the most critical parts of the system, without getting bogged down in implementing lots of rarely used features. I would constantly be looking for the highest value features to add or replace from the old system and use estimation as a tool to aid prioritisation of those.