You can manually promote the subscriber and all the other nodes will be attached to the new promoted publisher.
- First backup your current config
- Check the release notes or contact TAC to see if there's any known issues with current code about promoting a subscriber to a publisher , I was able to do in the latest code 6.5.5
- Go to Administration > Server Manager > Click on the subscriber and on the top right corner there's an option to promote the subscriber to publisher
In terms of downtime it depends on how much data it needs to sync from all the nodes , I suggest having an 2-3 hours downtime
You may need to contact support to re-activate licenses on the new publisher.