Do I need to user schemamigration and migrate commands after changing a field name or a field data type in a model?
Yes, Django won't recognize the field if you change the name.  I will say that the "field does not exist", so YES, you have to run Django's South migrate / schemamigration as you asked.  
Datatype YES as well.  Django may be okay at first if you only change the field type depending, but may run into problems later depending on what you have in that field.
The problem is pretty self-explanatory in the title. Do I need to do that or I just need to edit the existing migration file?