I' ve done 2 large software migrations in my life, here are my findings: 1. If you can't do gradual deployment, try to do a primary-secondary (master/slave) type of deployment where new system runs in read-only mode (mirroring old system data) for a while. 2. Whatever you budgeted for migrating data, double it. Set a data cleansing specialist to start working on the data-to-be migrated ASAP. 3. Document all processes of the current system. Have the painful conversations up-front about functionality that will be eliminated, migrating usually means eliminating a bunch of features that do not pass the cost/benefit threshold. Your users/stakeholders might not see it that way, make it explicit what the cost of those features is, get as much buy-in as possible