What is the procedure to change the data type of an existing field from dropdown to free text ?

We want to change the 'Salutation' field ,which is now a dropdown, into a free text field , so user can type in whatever saluation is applicable for their contacts. As an international company, having a dropdown for that would simply get too long (up to 50 variations per region...).

What's the easiest/safest way to change that field from drop down to free text ?

Thanks

Hugo