Here are the notes for upgrading nmdb from one version to another.

While normally nothing should be done, sometimes things change in incompatible
ways. Here's the listing for the releases, you should always check it before
upgrading.

0.20 -> 0.21
 * nmdb_get() and nmdb_cache_get() now return -1 instead of 0 when there is a
   missing key. This was done to allow setting a value to be "" (no data). All
   bindings have been upgraded, so you only need to worry if you used the C
   library.

0.21 -> 0.22
 * The protocol has been changed in an incompatible way. Do not use the
   0.21 library with the new server.
 * nmdb_incr() and nmdb_cache_incr() now have a new "newval" parameter. If the
   increment was successful, newval will be set to the resulted value.
   All bindings have been upgraded to reflect this change.