It's 1.5tb of data per indexer server, can Splunk handle that much data over 16 billion events..
Here you go (title says rename but it's actually migrating indexed data buckets to new index)
https://answers.splunk.com/answers/28134/rename-an-index-in-4-1-8.html
I am pretty sure you can't without reindexing counting against you. You can move it using the collect
command, but that will go against your license.
I think you should try reaching out to your Sales Rep to see if you can get a temporary increase or find out if the new licensing models will allow this as a one-day license error.