The reason you need to reimport your original files is that Solr creates an index by processing data extracted from those files. It doesn't necessarily store all of the original data. If you change your index schema, you need to go back to the original files since Solr usually doesn't have enough information in its index to recreate the original files so that it can index them in a different way.
In the case of VuFind's MARC-based indexing, you do actually have all of the original full records in your index, so you could theoretically extract them from the index before rebuilding the index rather than going back to your ILS. I believe that Mark Triggs' MARCgrep tool (https://github.com/marktriggs/marcgrep) might help with this approach. If you have indexed non-MARC records, you will most likely have to go back to the original source.
One important note, though: if you just want to reindex a certain set of records, you can reindex them without deleting the rest of the index, and the new versions will simply overwrite the old versions.
I hope this helps clear things up, but let me know if you still have questions... and if you tell us more about what you would like to do, maybe we can help you figure out a more convenient way to do it.
From: Thomas Misilo [mailto:misilot@...]
Sent: Thursday, May 24, 2012 7:27 AM
Subject: [VuFind-General] A way to reindex without deleted the solr cache and reimporting everything
I was wondering if there is a way to reindex everything without reimporting all the files?