How to change a Text field to a drop down field?

Hi,

I´m trying to change the "Primary address country" field, which is a text in the Leads module to a drop-down field.

I was able to do it with the "Billing address country" field, with the module loader. But when I do the same with the Leads module. It doesn´t work.

Does anyone know the best "recipe" for this?

Thank you in advance.