Hi,
I didn't understand where you can't fill the new field value for the old forms,if it is in the share point site promoted fields then,
for that you have to check the "Allow Over write if file exits" option while publishing the template. By this option the new form is resubmitted by updating/overwriting the old form including in the share point promoted fields also.
Note:I think you can fill out the new field value in the InfoPath form because you are saying as you can fill that value for the new fill out forms. So, you can definatly overwrite the new field for the old forms also, until you didn't use any conditional formatting or rules for that new field .
I hope the given information is useful to solve your problem.