I suspect the "zero down time" requirement is unrealistic. There has to be a time where all transactions on the old system are finalized and the data is frozen for migration. Then it has to be translated and moved to the new system, and validated there before you let the world in. At the extreme least there will need to be several minutes where the data isn't available, assuming you move the majority of the historical data in advance and then migrate the final day/hour at Go Live time. I've been involved in two migrations, and both had over four hours of downtime (and we were delighted to get it down to that). Whatever process you decide on, practice multiple run-throughs before go-live day.