Can a single node data repository be upgraded to the 3+ node cluster repository?
I think it can be done , please check the below
Add a Node to the Data Repository Cluster - CA Performance Management - 3.6 - CA Technologies Documentation
one caveat is if you are expanding from one node, you need to segment the database after expansion is complete
Segment Database Tables - CA Performance Management - 3.6 - CA Technologies Documentation
Anyways I have not done it practically anytime but as per the documentation, it does sound possible. So, I will leave it for further advice from someone who has tried it