There are a multitude of ways to accomplish this.
1) You can just create two fields, 1 for Original EED, 1 for edited EED. Set a form rule that says, "If [Original EED] is not blank, make [Original EED] read-only.
2) You can create an [EED] field, and then create another field called [EED Snapshot]. Open the [EED Snapshot] field and under the settings, near the bottom there is a field that says "Snapshot: Get this field's value from another field and don't allow it to change." click that and select the [EED] field as the source for the value. Make sure to initialize fields if applicable.
3) Create two fields, say [EED] and [EED Snapshot], create a form rule that says "When [EED Snapshot] is blank and When [EED] is not blank, change [EED Snapshot] to the value in the field [EED]. Keep the "fire on change.." checkbox checked.
4) several ways to automate as mentioned already.
I recommend using the native snapshot functionality whenever possible, or hard coding it as in example #1