ElasticSearch will guess at they type of field at the creation of each Index, I am reasonably sure that OpenSearch is the same. If you rotate the index, it will choose type anew… so if you are forcing a long, it should capture as a long on the creation of the next index.
Assuming that works, It is possible to correct a field to a long historically if you want… Here is a write up for an older version but it likely can be used in OpenSearch with a little tweaking.