The minimum database version that can be  converted  is  version
  <num>.

  Explanation:  Your database version is too old to be converted.

  User Action:  You must convert your database using 2 versions of
  Oracle  Rdb.   First  convert  your  database  using the minimum
  version of Oracle Rdb specified in the error text.  Next convert
  again using the version of Oracle Rdb that reported the original
  error.  For example, Oracle Rdb V7.0 can only convert Oracle Rdb
  databases  V5.1  or  greater.  If you are trying to convert from
  4.2 to 7.0, then you must first convert using Oracle  Rdb  V5.1,
  V6.0  or  V6.1.   Then  you can convert the database again using
  V7.0.