To do this online will mean that your users wont have access to the archive while the indexes are being rebuilt
There is another option but it requires alot of manual work
Symantec came up with a process to rebuild indexes offline. It requires setting up a temp environment where the indexes can be moved to, rebuilt and then moved back into the live environment and updated.
This process they wrote for my company because we have many TB of indexes and they are all unstructured. These take a week or more to rebuild so they gave us a way to do it offline
Not sure if this is a solution you would be looking for
