There exist type systems that can handle "strong" (i.e. type-changing) updates, but the ones I know of will only permit it for unique (linear/affine) references

There exist type systems that can handle "strong" (i.e. type-changing) updates, but the ones I know of will only permit it for unique (linear/affine) references. You might be able to take advantage of the guarantee that properties are not being removed, so the update just turns the object into a subtype of what it once was. Then other pieces of code with reference to that object can continue on type-safely, but they won't be able to use those new properties without somehow being informed about them (statically).