Base subobjects cannot be transparently replaced with complete objects, as specified in 6.7.3 [basic.life] bullet 8.4:

An object o1 is transparently replaceable by an object o2 if:

• ...
• neither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), and
• ...

However, that bullet is over-reaching, because it disallows:

  struct A { int n; };
  struct B : A {};
  B b;
  new (&b) B { {5} };  // New A base class does not transparently replace existing A base class due to /8.4.
  int k = b.n;  // UB: member n of A base class is outside its lifetime

See issue 2677 for a suggested resolution.