Once RE1 is successfully upgraded to the targeted version and after enabling GRES, NSR and commit synchronization on RE1, if we try to switchover mastership from RE0 to RE1 we get the switchover-not-ready error logs.

Below output will be observed stating the reason on why the switchover is failing.

labroot@MX> Graceful switchover: On

Configuration database: Ready

Kernel database: Version incompatible

Switchover Status: Not Ready

We can perform below action:

• A network-services mismatch alarm will  triggered due to version incompatible between RE 0 and RE 1, due to which the switchover will fail
• We have to disable the GRES, NSR, nonstop-bridging and commit synchronization which is configured on RE 1.
• Perform the master-ship switchover (RE 0 ->RE 1)
• Upgrade RE 0 to the desired version
• After rebooting RE 0 the network-services mismatch alarm will be cleared
• Enable the GRES, NSR, nonstop-bridging and commit synchronization, 
• Perform the RE master switchover from RE 1 to RE 0.