1. Run upgrade from 4.5 to 4.6
Keepalived will continue to tun in multicast mode
2. Add a new worker node
The new node run Keepalived in unicast mode
The new node should run Keepalived in the same mode as rest of the nodes
The fix for this bug was already merged, see 
Client Version: 4.6.0-0.nightly-2020-10-03-051134
Server Version: 4.5.13
Kubernetes Version: v1.18.3+47c0e71
Worked by procedure  to verify.
After adding the worker-0-2,
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (OpenShift Container Platform 4.6 GA Images), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.