Context: We have had 3 customer cases for the upgrade error. 

Please add the following to Serverless troubleshooting guide,

Q: Why am I not able to upgrade my Serverless Operator? Error message : 
Error","message":"Version migration is not eligible with message: not supported to upgrade or downgrade across the MAJOR version
A: This error message means there was a skip in a version (for whatever reason) (could be manual uninstall of an old version of Serverles + replaced by a new version of Serverless)So, to fix it, the best way would probably be:

1. Identify the proper version of the Serverless operator that was originally installed, if the message says The installed KnativeServing version is v0.25.1.  , that means Serverless 1.19 ,  see the branches and the metadata in https://github.com/openshift-knative/serverless-operator/blob/release-1.19/olm-catalog/serverless-operator/project.yaml#L25  , or https://docs.google.com/spreadsheets/d/1HTxR37_MM03_JQImW-KmAFk1oEQ83fXzfGxdfWQSX6o  (note that the knative micro versions are not always exact)

2. Uninstall the serverless operator and any of its installplans
3. Manually install the original serverless operator version, preferably with a manual approval
apiVersion: operators.coreos.com/v1alpha1
kind: Subscription
metadata:
name: serverless-operator
namespace: openshift-serverless
spec:
channel: stable
installPlanApproval: Manual
name: serverless-operator
source: redhat-operators
sourceNamespace: openshift-marketplace
startingCSV: serverless-operator.v1.19.0
once the knativeserving is ready,  upgrade by manually approving the upgrade installplans once they appear.
 
//cc maschmid@redhat.com rhn-support-afield 

Update: Instead of a trouble shooting section, we need to add an Upgrade section to our docs that would take care of these customer cases.