We do this too and I think it's a great approach. One thing I will say is you should try and decommission the old system as soon as feasible - running both has

We do this too and I think it's a great approach. One thing I will say is you should try and decommission the old system as soon as feasible - running both has additional testing and development costs, particularly if for some reason you need to add new features to both. We generally aim to leave the old system toggled off for a release or two (allowing us to switch back in the case of a serious defect) and then rip all that old code out in the subsequent release.