Reindexing indexes all documents again and therfore rebuilds all data structures on disk and in memory. Reallocation on the other hand generally just involves moving aleready computed segment files between nodes which requires a lot less computational work and is a lot more efficient, but can not be done between clusters.
Ah, good point. If you have large data volumes to move you could look into using snapshot and restore instead as that moves segments and do not require reprocessing.
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