backward compatibility

[袖宏治]

Hello, it is sode.
I created an index in lucene 6.1. I want to use the index created in lucene
6.1 with lucene 6.6.
In lucene 6.6 I looked up index in lucene 6.1. The following Error has
occurred.
Could not load codec 'Lucene 60'. Did you forget to add lucene - backward -
codecs.jar?
  I put lucene-backward-codecs.jar in library Path, but it got similarly
Error.
Is the index of lucene 6.1 unusable in lucene 6.6?