Keep Coding

I have a production database and need to keep safe the data. I want to change a Field in model and convert all data inside that database with this change.
Old field
class MyModel(models.Model):
field_name = models.TimeField()
Changed field
class MyModel(models.Model):
field_name = models.PositiveIntegerField()
Basically I want to convert the TimeField value (that has a Time object) in minutes.
Example: I have in an object time(hour=2, minute=0, second=0) and I want to convert that field value in all database table to 120 when I apply the migrate.

Safest way that I always do is:
create another field with field_name_new = models.PositiveIntegerField()
migrate this new field on production db
do data migration (convert field_name value and move the
converted value to field field_name_new)
change your code so that field_name_new is used
deploy the step 4
delete the field field_name
migrate the step 6 on production db and deploy it (the step 6)
there is only one bad side: you may loose some data in steps 4 and 5. but you can replicate these data to new field basically

In the past, I've always done this with the help of Djangos RunPython operation. Create a custom migration that handles the following.
Change name of field.
Add new field of desired type.
RunPython to handle the logic of converting from one to the other.
Delete old field.
def migrate_time_to_positive_int(apps, schema_editor):
MyModel = apps.get_model('myapp', 'MyModel')
for mm in MyModel.objects.all():
field_old_time = mm.field_name_old
field_new_int = field_old_time.total_seconds() / 60
mm.field_name = field_new_int
mm.save()
class Migration(migrations.Migration):
operations = [
migrations.RenameField(
model_name='mymodel',
old_name='field_name',
new_name='field_name_old',
),
migrations.AddField(
model_name='mymodel',
name='field_name',
field=models.PositiveIntegerField(),
),
migrations.RunPython(migrate_time_to_positive_int),
migrations.RemoveField(
model_name='mymodel',
name='field_name_old',
)
]
field_name_old.total_seconds() / 60 might need to be adjusted, but you get the idea.