Simple is Best

sorry my english.

if my user model have

t.string :name
t.string :age

and data

1 : John / 34
2 : Tom / 25

if I want to add a field and change field name

t.string :english_name       #change field name
t.string :age
t.string :location                  #add field

rake db:migrate - not work.

rake db:drop
rake db:migrate - working. but John and Tom is gone. RIP

How can i keep data while "add field" or "change fieldname" in model?