How to change field_tags in hook_form_alter?
The field_tags element is likely a widget from the Field API. You can surely alter it from a form alter hook, but be aware that it would not be your average Form API element. The Field API adds post-processor, pre-rendered, etc. functions of its own. These functions may expect things you may want to alter.

You can however, change the default value of the field from the configuration interface of your content type (you need to enable the Field UI module for this). You should also be able to select a different widget (ie. a select box) from there.