Allow other index types to be updated to disk_bbq

### Description

once the format of disk_bbq is no longer behind a feature flag, we should allow other index formats to be merged into disk_bbq allowing for updates. 

My intuition here is that we allow updates from *flat, *hnsw, -> disk_bbq. 

Additionally, I would also think we should allow `disk_bbq` updates to particular settings (e.g. target cluster size).