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.
