In 3.9.0 we added 64 bit indexes. If you are currently upgrade all
your new indexes will be created as 64 bits. Your old indexes will
remain as 32 bit. If you prefer to have 64 bit indexes replace your
old indexes there are a couple of ways of doing this.
For a small db just delete the index folder and let it rebuild (might
take a while)
If you have a large db / remote storage / etc and cannot take the time
you can also do this operation offline on another node:
1) Take back up.
2) Put on fast local disks
3) Let it rebuild
4) Restore the index back to a node (index folder)
5) Let it catch up from master.
6) Repeat 4/5 for other nodes.
For others the index will eventually be 64 bit due to the merging
process that occurs over time.