does Bitcoin Core automatically use reindex when you specify txindex=1 with existing sync'd chain?
Changing the txindex setting no longer requires a reindex since version 0.17.0 (see https://bitcoincore.org/en/releases/0.17.0/, under &quot;Transaction index changes). It'll update just the txindex itself whenever it's out of sync automatically.
So:

It doesn't inject a reindex (which would cause throwing away the entire chainstate database, and rebuilding it). It just detects the txindex is out of date, and scans the unindexed part of the chain in the background.

The command is &quot;bitcoin-qt&quot;, almost certainly, with nothing more.