I expect that it is rather likely that new job names will be similar to old job names, so would keep the default value to make minor changes easier. If you could detect it's the current job's name, just showing nothing, or an info message "same name" would be useful.
EDIT: I should clarify, do you think renaming to the same name should succeed as a no-op, fail, or do something else?
Not sure. Either could work. Since form submission that would change nothing is unlikely to be intended, it should probably remain an error.
danielbeck what do you think about just leaving the name blank by default on that page? You'd still get an error ("No name is specified"), but since you have to pick something for the new name anyway that doesn't seem like a problem.
I can also improve the error message in the case where the name is the same as before to "The new name is identical to the current name".
EDIT: I should clarify, do you think renaming to the same name should succeed as a no-op, fail, or do something else?