Recently upgraded elasticsearch and kibana from 6.x to 7.0.1. I checked out the "8.0 Upgrade Assistant" from the Management section. It reported that my index was created with an older version, and suggested I reindex after a backup, which I performed.
During the process, it flawlessly recreated my index, using aliases to ensure functionality. However, I had already recreated the index myself in the past, adopting a naming format of indexname_vn,
where n indicates the version. (This was due to the need to change a field from auto-discovered integer to float, a relatively common problem newbies like me might encounter.) I then manually create an alias indexname to reference the original usage before adopting the naming format and thereby preserving functionality.
In this case, the automatic index recreation muddied the waters a bit. Instead of using indexname_v[n+1], as I would have preferred if I were able to make the decision myself, it created an index with name reindexed-v7-indexname_v2, with aliases indexname and indexname_v2.
The end state I would have preferred is to simply have indexname_v3 with a single alias, indexname.
If I were able to select the destination index name, and perhaps alias names as well, the end state would have been cleaner. To be clear, there appears to be ZERO functionality loss at this time, and this is essentially cosmetic.