class Entry(models.Model):
owner = models.ForeignKey(User, related_name='owner')
myfile = models.FileField(upload_to='my_files/%Y/%m/%d')

import os

    def save(self, *args, **kwargs):
        if self.pk:
            prev_instance = Entry.objects.get(pk=self.pk)
            if prev_instance.myfile:
                 name = os.path.splitext(prev_instance.myfile)[0]
                 os.rename(prev_instance.myfile, name + ".old")
        super(Entry, self).save(*args, **kwargs)