<input type='submit' value='Submit' onclick='this.form.action="somethingelse";' />
Or you can modify it from outside the form, with javascript the normal way:
document.getElementById('form_id').action = 'somethingelse';
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…