Although this is a new major version release it is fully compatible with former versions. There is no need to re-index your collection.
Solr 5.x support
The main reason for the major version is the switch to the new Solr 5.x version branch. pixolution flow is now compatible with the current Solr 5.2.1 version and all its cloud features. We refactored our architecture and internal code structure and rewrote most of our unit test framework.
From a user perspective there are no changes to the pixolution plugin API, there is no need to rewrite your query code.
Solr 5 also introduced some changes to the configuration files
Beside the changes mentioned there you do not need to change your pixolution flow configuration when upgrading from Solr 4.x to Solr 5.x.