As of now , if a Jira-admin/roadmap user is not granted Edit permission on a Plan level , this restriction cannot be changed by the jira-admin/roadmap admin users. 

Steps to replicate:

• Log in as a Trusted user , who is not a Jira-administrator, and does not have Administrative permission for Advanced roadmaps.
• Now with this Truster user , set the Access control ->Permission for editing to this user alone.
• Switch back to your account ( who has permission to administer Jira /Advanced roadmaps).
• The Plan which was restricted for Editing will be Read-only for you. Jira-admins/roadmap admins cannot change the Access control Permission.

Suggestion:

It would be good if the roadmap admins can change the restrictions set on Plan level by other admins or users. This is somewhat similar to Private plans not being accessible to Jira admins , https://jira.atlassian.com/browse/JSWCLOUD-20187.