when your Branch completely converted into SDWAN, you don't have any other option to make tunnel establishment between SDWAN and NON-SDWAN components (Overlay). either you need to setup both SDWAN and NON-SDWAN in your branch too or you have to use another site to be used for this failover and that site should have both legs (SDWAN and NON-SDWAN). So, you can communicate from SDWAN sites to DC via intermediate site which has both Leg.