It's 1.5tb of data per indexer server, can Splunk handle that much data over 16 billion events..
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.
Here you go (title says rename but it's actually migrating indexed data buckets to new index)