Requirements version 1.0
need return {"name": "JohnDoe"}
=== API version 1.0 ===

@Entity
class Model {
   @Column
   String name;
}

@Json
class ModelJson {
   String name;
}

@Service
class ModelService {
   Dao dao;
   ModelToJsonMapper mapper;

   @Transactional
   ModelJson getModelJsonBy(id) {
      Model model = dao.getBy(id);
      ModelJson json = mapper.toJson(model);
      return json;
   }
}

Requirements version 1.1 (!)
need return {"name": "JohnDoe", "qty": 2}
=== API version 1.1 ===

@Entity
class Model {
   @Column
   String name;   // ! remains the same
}

@Entity
class SomeQty {   // another entity
   int qty;
}

@Json
class ModelJson {
   String name;
   int qty              // ! field added
}

@Service
class ModelService {
   Dao dao;
   ModelToJsonMapper mapper;
   SomeQtyService qtyService;   !!! EVIL solution No1

   @Transactional
   ModelJson getModelJsonBy(id) {
      Model model = dao.getBy(id);
      SomeQty qty = qtyService.getBy(...);  !!! EVIL solution No2
      ModelJson json = mapper.toJson(model, qty);  !!! EVIL solution No3
      return json;
   }
}

_Model_Service (!! not Qty) became dependent on other service of other entity, not even connected with each other. Reason is just miserable json changes. Conclusion - you are NOT controling your internal architecture. It is out of your own control. Json format rules over your service architecture instead of you.
You've increased service layer complexity.
You've increased services strong coupling.

What it MIGHT be:

=== API version 1.0 ===

@RestController
class Controller {
   ModelService modelService;
   ModelToJsonMapper mapper;

   @Request
   JsonModel getBy(id) {
      Model model = modelService.getBy(id);
      ModelJson json = mapper.toJson(model);
      return json;
   }
}

=== API version 2.0 ===

@RestController
class Controller {
   ModelService modelService;
   QtyService qtyService;       // 1 !!!
   ModelToJsonMapper mapper;

   @Request
   JsonModel getBy(id) {
      Model model = modelService.getBy(id, FULL);
      SomeQty qty = qtyService.get(..., FULL);     // 2.1
      ModelJson json = mapper.toJson(model, qty);  // 2.2
      return json;
   }
}

1 - Presentation layer became dependent on service layer, not vice versa, in opposition to your approach.
2 - json changes affects ONLY presentation layer complexity, and NOT a service layer complexity.