1) change in cardinality is such a change in domain that lenses won’t solve it. There will be much bigger changes in business logic probably making original code obsolete regardless of used pattern.
2) deep tree modifications of the kind that you mentioned indicate problems with architecture. Why would you need the whole typed tree (not DOM or something, but object tree) to modify a tiny leaf of it? If you touch multiple leafs with root as closest integration point, why your data model is designed like that, pointing to strong coupling of different contexts?
3) most importantly the code that can be reused, can be extracted to a private method of an entity where the change occurs. Calling nested constructors on the root object breaks encapsulation — it should pass the message to nested objects instead.