@John Huebner, thanks. That seems to work. However, this prev_value is needed by another page. So is it possible to save this prev_value in a field which is purely to make it available to other pages. It shouldn’t be editable (preferably not even seen but at least disable/un-editable).
A possible solution I got in mind is to create a separate field that holds the last non blank value that’s gets updated using update_field(). However, I don’t want this field to be editedable by admins.