What is the best way to reindex only models that have changed during the solr downtime?

if I have millions of user records with some text fields that are indexed in solr when creating and updating, how can I go back and reindex several records that never made it in solr?

i.e. what if the solr drops by a few minutes during the day, and about 300 records out of millions are never indexed.

I do not want to re-index millions of records, only 300.