Currently, we don’t support $merge on time-series collections. We are aware of this limitation and have it on our roadmap as something we want to support in the future. $out is currently the recommended way or custom script might make sense in your case.
For $out, you would have to transform the data in your regular collection into new desired TS schema in a temporary collection and then mongodump/restore can be used on the transformed collection created from the $out output.
You can find more information on the steps here – https://www.mongodb.com/docs/v5.3/core/timeseries/timeseries-migrate-data-into-timeseries-collection/.