Migration from IdentityServer4 to AzureB2C

We've explored this but are evaluating if all the custom features we've implemented can be recreated in B2C without it costing the earth in dev effort.

I think the best migration approach is to have one of them (your existing IDS4 IDP or B2C) temporarily act as a federation gateway for the other. This means your client applications don't have to worry about deciding dynamically which one to use (which I'm guessing your second question was leading to).

It probably makes sense to make this your IDS4 service since you can taylor the logic more freely and route users appropriately depending on their migration status.

Once the migration is complete you can remove your IDS4 instance from the mix and have your client application(s) go direct to B2C for authorize requests and token validation etc (with some reconfiguration involved ofcourse)

There's a good writeup here around migration.

In this scenario, you point all apps to B2C and B2C figures out who to authenticate against.