Migrating a regular data stream to be a tsdb data stream should be achieved by
setting the index_mode parameter to time_series in the data stream snippet
of the composable index template that created the data stream. This should only
be possible when no index_mode has been specified in the composable index template.
When this change has been made to the composable index template, then the next
time the data stream rolls over the index_mode property of a data stream will
be set to time_series and a new backing index with index.mode index setting
set time_series will be created. All new backing indices created by rollover
for this migrated data stream will be tsdb indices.