For dwngrade and 2dsphere indexes:

If you have version 3 2dsphere indexes (i.e. the default version for 2dsphere indexes in MongoDB 3.2), drop the version 3 2dsphere indexes before downgrading MongoDB. After the downgrade, recreate the 2dsphere indexes.
To determine the version of your 2dsphere indexes, run db.collection.getIndexes() to view index specifications. For 2dsphere indexes, the method returns the version information in the field 2dsphereIndexVersion. For example, the following shows that the 2dsphere index on the locations collection is version 3.

ditto for text index version 3