You can safely delete the contents of article_search_keyword_list, article_search_object_keywords, and article_search_objects before upgrading – these tables contain the full-text search index and can be re-generated afterwards by running php tools/rebuildSearchIndex.php.
However, it’s worth checking your character set configuration to make sure you aren’t accidentally changing something; make sure those parts of your config.inc.php match your old version.
Regards,
Alec Smecher
Public Knowledge Project Team