Hi
Yes it is not possible to change only a field to editable,
Rather what you need to do is to set the "On Validate" code of the field in the form or page to check if it is allowed to be edited then generate an error if not, the error will cause the system to empty the field again
Field - on validate()
IF Rec.OtherField = '' THEN
ERROR('you cannot fill in this field if the other one is blank');
Hope this makes sense
Nev