[Segment Replication] Support enabling/disabling segment replication for an existing index.

[mch2]

Is your feature request related to a problem? Please describe.
Segrep right now can be enabled only during index creation. This means converting an existing index to use segrep requires a full reindex.

Describe the solution you'd like
Support converting an existing index to use segment replication without reindexing.

Describe alternatives you've considered
None

Additional context
Add any other context or screenshots about the feature request here.