Hi Maja_B,
As my colleagues said, the SOLR version was upgraded in Version R81 and above, I would recommend for an upgrade from R80.xx versions to use (migrate_server export -v R81.xx -l <file name>) command to export only the log files not including the indexes.
this will transfer all logs under $FWDIR/log to the new version, than you may increase the days to index backwards as much days as you desire, you can track the index process with the following command (watch -d 'cat $INDEXERDIR/data/FetchedFiles').
to increase the days to index, just follow this simple steps:
1) cp $INDEXERDIR/log_indexer_custom_settings.conf $INDEXERDIR/log_indexer_custom_settings.conf.ORIGINAL
2) vi $INDEXERDIR/log_indexer_custom_settings.conf
3) add under ":max_disk_space_usage (0)" the following ":days_to_index (<number of days>)" then save changes
4) run 'stopIndexer' & 'startIndexer'
Kind regards, Daniel Fidlin