Eh, sayang lah kalau tak guna nanti expired kan, tapi kalau baby dah happy dengan Similac, honestly tak perlu force switch lah, babies sometimes boleh jadi fussy if you change formula suddenly. Maybe boleh try la slowly mix sikit Enfalac with the Similac, or simpan je for emergency/backup, sebab free pun so tak rugi! A few of my mum friends do this, they keep one tin of different brand just in case habis susu or if they want to see if baby boleh tolerate other brands.