Select-list cascading CF: how to make the first value of the field read-only (and not the 2nd) ?

[NSI Atlassian global account]

Hello,

For one of your client, we've created a cascading select-liste custom field in a jira service desk project. 

One the ticket has been created, we would like the agent to be able to modify the second part of the CF, without being able to modify the 1st part.

How can we do it ?

Thank you