To add to this, your DS / LM / HF can be taken down with out and effect to your search or indexing tier (aside from the associated data ingestion.) So typically, the maintenance windows on those roles can be whenever.
You do need to pay attention to version compatibility. Between forwarders and indexers typically isnt a issue, but the search tier to indexing tier does have some constraints..
Forwarder Requirements and Compatibility -
http://docs.splunk.com/Documentation/Splunk/6.3.3/Forwarding/Compatibilitybetweenforwardersandindexers
Distributed Search Requirements -
http://docs.splunk.com/Documentation/Splunk/6.3.3/DistSearch/Distsearchsystemrequirements
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