if the taxon change isn’t handling this automatically, it is possible to use the API to withdraw a given set of IDs, using a process similar to the process noted in this post and subsequent posts in that same thread (related to batch adding observations to a project): https://forum.inaturalist.org/t/bulk-selection-of-observations-to-add-to-a-project/1747/70.
the relevant template command for withdrawing an identification would be this:
curl "https://api.inaturalist.org/v1/identifications/%identification_id%" -X "PUT" -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: %jwt%" -d "{\"identification\":{\"current\":false}}"
and suppose this was the set of identifications you wanted to withdraw: https://jumear.github.io/stirfry/iNatAPIv1_identifications?user_id=charlie&taxon_id=470643&active=true&per_page=200
… then this would be the template for getting a list of identification ids: https://api.inaturalist.org/v1/identifications?user_id=charlie&taxon_id=470643&active=true&per_page=200&only_id=true