This is now supported in Sitecore versions 8.2.7 and 9.0.2 and later.
From the link below:
A full index rebuild executes in a separate rebuild index. When the
rebuild operation finishes, Sitecore switches the Search service to
use the new rebuilt index as the primary search index. After this,
Sitecore deletes the previous Search index.
You can control the Switch On Rebuild feature with the following index configuration settings:
<setting name="ContentSearch.Azure.SwitchOnRebuild" value="true"/>
To avoid downtime of the Search functionality during an index rebuild, use this setting to specify whether the Azure Search provider uses the SwitchOnRebuild functionality. The default value is true.
<setting name="ContentSearch.Azure.SwitchOnRebuildCleanUpDelay" value="00:01:00"/>
You can set a time buffer to prevent Sitecore deleting the old index before all of the roles have been switched over to work with the newly rebuilt index. The default value is 00:01:00 (1 minute). You can only use this when the SwitchOnRebuild feature is enabled.