I would recommend you set up a completely new data stream and start writing any new data into this. You should then be able to add an alias on top of the large index and use this directly or as part of an index pattern to query the new data stream and the existing index together. Once all data in the current index has exceeded the retention period you can delete it and rely on just the data stream.
If you want to get rid of the current index earlier you may reindex some part of the data in this index into the data stream (sort by timestamp to get documents in the correct order) and then start ingesting new data into the data stream.