I’m going to presume here that one of the goals of migration is to be completely independent of the “previous system”, just in case it disappeared forever.
So (hypothetically)…if “something happened” to FP Complete, the old URL would be broken permanently: there also goes the “new user experience”. To me, the “middle option” is to keep the old URL around until support of the final pre-migration version of Stack ends: then the old URL can be deleted.
Now if at that time there are still people using pre-migration versions of Stack, then they’re are either very overdue for an upgrade anyway, or they will have to learn how their version of Stack works so they can support it themselves (in much the same way someone using an old version of HBC, NHC, JHC, Hugs, or even Yale Haskell would have to). I doubt very few of them would still be new Haskell users…