On the need to reindex after upgrade

Looking for guidance, general tips about when should we reindex on upgrade.
According to posts When do I need to reindex data?https://groups.google.com/d/msg/elasticsearch/QtRD_4eJjig/boAX0TnvovkJ,

  • Lucene is backward compatible within one version
  • Lucene 4.0 has an emulation layer that makes it safe to work with
    version 3.
  • "full cluster restart" in release notes means we just need to stop the
    whole cluster, then index will be opened on restart ... no need to re-index

So, what are your suggestions/tips on this? Any specifics on Lucene/ES
version diff ? Any section or warning in Release notes to look for?
Maybe a FAQ entry covering this topic?

Thanks in advance,

p.s.: I will be upgrading from 0.20.5 to 0.90.5, and have no plan to change
index settings, type/fields mappings, nor use new features.
We will be testing the old index once upgraded, any tips you have in mind?

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