change field type

You cannot change the field type of an existing field. If the field is an existing field, you would need to delete it and create a new field. If the new field needs to be able to pick up existing data from existing users using that field, then make sure to use the same meta key. Otherwise, the meta key doesn’t matter.

Keep in mind also that the fields the plugin installs by default can be deleted and do not have to be used – they are simply there as a starting point and/or example. There’s no requirement to use those particular fields.