My baby turning 12 month soon and has been having diarrhea for almost 11days.

I really feel you on this, 11 days is such a long time and I can imagine how worried and exhausted you must be. Before switching milk, I would strongly encourage you to bring your baby to see your PD as soon as possible, because diarrhea lasting that long in a child so young really needs to be properly assessed first. The PD will be the best person to confirm whether it is a lactose issue and to recommend the right formula for your baby's specific needs, rather than us guessing from here. Please do not wait too long, just to be safe lah.