There is a possible third approach, in addition to the two suggested by @RavinKumar and @OriolAbril. In a post about applying an existing model to new data, @lucianopaz suggested creating a new model with a model factory. I think his model factory approach could also work for solving this problem: create a new model using the existing data and fit to newly altered posterior samples of the old model.
Note: I haven’t tried this approach.