When you Migrating from one vendor to other vendor i am sure you will have small downtime need to be agreed when you move the Physical connection end device to new IE Switch.
I prefer to do this way, Build new IE Ring and in the Main Location connect one of the IE Switch to Other Ring network to enable connection between Old ring network to new Ring network - start Migrating the Devices in each Location - start with less impacted device and Monitor the network stability before you Migrate rest of the devices.
Hope this helps you.
