Move 40 branches from MPLS to SD-WAN without breaking site-to-site traffic

A retailer has 40 branches and one data centre on an MPLS L3VPN. Each branch CE runs eBGP with the provider and advertises its LAN. The company is adding a business internet circuit at every branch and moving to SD-WAN, keeping MPLS as one of two transports for at least a year. Branches will migrate in waves over three months. Throughout, migrated and not-yet-migrated branches must reach each other and the data centre, and card payments and voice must not suffer.