Currently if you have a string column that have only number values (think a year column using string by mistake) and you want to change to integer, you can’t.
If you apply this monkey-patch will be possible: https://gist.github.com/1393441
Note: if some value can’t be casted by postgresql (if have a letter for example), the migration will fail as expected.
I think this is the expected behavior on change_column and I’m not sure but I guess mysql already does that.
What you think?