Currently it is possible to migrate an existing data stream to a time series data stream.
This is done by updating the index template that created the data stream. 
Then after a rollover the data stream is a time series data stream. 

Via a similar way it should be possible to change time series data stream back 
to a regular data stream. Currently when changing the `index.mode` setting
on an index template from `time_series` to `standard` (or removing the setting)
results in a validation error.
server/src/main/java/org/elasticsearch/cluster/metadata/DataStream.java
Strings.collectionToCommaDelimitedString(im.getAliases().keySet().stream().sorted().toList())
                )
            );
        }
        List<Index> backingIndices = new ArrayList<>(indices);
        backingIndices.add(0, index);
        assert backingIndices.size() == indices.size() + 1;
        return new DataStream(name, backingIndices, generation + 1, metadata, hidden, replicated, system, allowCustomRouting, indexMode);
    }
    public DataStream promoteDataStream() {
        return new DataStream(name, indices, getGeneration(), metadata, hidden, false, system, timeProvider, allowCustomRouting, indexMode);
    }
    /**
     * Reconciles this data stream with a list of indices available in a snapshot. Allows snapshots to store accurate data
     * stream definitions that do not reference backing indices not contained in the snapshot.
     *
     * @param indicesInSnapshot List of indices in the snapshot
     * @return Reconciled {@link DataStream} instance or {@code null} if no reconciled version of this data stream could be built from the
     *         given indices
     */
    @Nullable
    public DataStream snapshot(Collection<String> indicesInSnapshot) {
        
        List<Index> reconciledIndices = new ArrayList<>(this.indices);
        if (reconciledIndices.removeIf(x -> indicesInSnapshot.contains(x.getName()) == false) == false) {
            return this;
        }
        if (reconciledIndices.size() == 0) {
            return null;
        }