An error occurred while validating the certificate: The certificate chain provided is not valid

First, you should go to new appliance and note its ip address, at the migrate in Step 2, you should enter old appliance current ip address and enter credentials and Click on "Trust Source-Appliance Certificate" if this confirms, 
Go back to old appliance and Validate connection with new appliance ip address and credentials.
Then go furher steps 3 check "Terminate running tasks" and run this step.
When 4th step completes, it directs you to new appliance migrate in process. 
At new appliance now "Validate Connection", if it gets Success as result this will lead you to begin import process.

This is not described simply at Article.