Recently I had to upgrade someone's Apache Solr installation from 1.4 to 5.x (the current latest version), and for the most part, a Solr upgrade is straightforward, especially if you're doing it for a Drupal site that uses the Search API or Solr Search modules, as the solr configuration files are already upgraded for you (you just need to switch them out when you do the upgrade, making any necessary customizations).
However, I ran into the following error when I tried loading the core running Apache Solr 4.x or 5.x:
org.apache.solr.common.SolrException:org.apache.solr.common.SolrException: org.apache.lucene.index.IndexFormatTooOldException: Format version is not supported (resource: MMapIndexInput(path="/var/solr/cores/[corename]/data/spellchecker2/_1m.cfx") [slice=_1m.fdx]): 1 (needs to be between 2 and 3). This version of Lucene only supports indexes created with release 3.0 and later.
To fix this, you need to upgrade your index using Solr 3.5.0 or later, then you can upgrade to 4.x, then 5.x (using each version of Solr to upgrade from the previous major version):