Xapian versions usually support several formats for index storage. A given major Xapian version will have a current format, used to create new indexes, and will also support the format from the previous major version.
Xapian will not convert automatically an existing index from
the older format to the newer one. If you want to upgrade to the new format, or if a very
old index needs to be converted because its format is not supported any more, you will
have to explicitly delete the old index (typically
~/.recoll/xapiandb), then run a normal indexing command. Using
recollindex option -z would not work in this
situation.
