We're upgrading Grid Infrastructure from 18c to 19c on a 2-node RAC. We ran once without any problem except that after node 1 was done, the upgrade process immediately started to do node 2. We were not given a pause to relocate database services (therefore user connections).
https://docs.oracle.com/en/database/oracle/oracle-database/19/cwlin/understanding-rolling-upgrades-using-batches.html
talks about splitting RAC nodes into batches or groups. So after you upgrade batch 1, you have a chance to relocate DB services to the node(s) in batch 1, and you go on to upgrade batch 2, and so on. But I can't find how to define batches anywhere in documentation or any MOS document. We basically followed the procedure in
https://docs.oracle.com/en/database/oracle/oracle-database/19/cwlin/running-oui-to-upgrade-grid-infrastructure-from-an-earlier-release.html
[Update] Never mind. There is the screen "Select batches for nodes". The Admin can mark this problem solved or delete it.