I want to edit analyzer setting of an exist index to add a lowercase filter, should I rebuild the index? or there is someway easier.
thx~
For now there is nothing easier. You do a scroll query that hits all the documents and then re-add them to another index with the analysis setup the way you want so you don't have to rebuild from your source application - but only do that if your source application is slower.
If you don't have a source application - like if you are doing logging data or something, the normal practice is to add the change in analysis to the template for creating the next day/week/month's index and just live with the difference until your data retention period has you deleting the old indexes.
While I understand why modifying existing analyzers doesn't make sense, I'm curious why adding new ones requires closing and opening the index. I've skimmed through the code and couldn't find the answer. I suspected that maybe Lucene requires all analyzers to be defined when the IndexWriter is being instantiated but it's not the case. I'd appreciate any help with this 
It might have to do with the way analyzers are registered:
Ivan