Making an EFF read only after upon edit

In RUI, We need an EL Expression to make a field in the Additional Assignment Information as read only after initial creation ( After adding)

Content (please ensure you mask any confidential information):

We have a requirement wherein we need to make a field in the Additional Assignment Information as read only after initial creation (After adding details) upon Edit.

We need to have the "Bei Firma" field as read only upon edit.

Currently the behavior via page composer is if mark the field of the EFF as READ Only, it is marked as read only also upon adding the initial details.