We where told it's up to us as my boy hated any kind of milk but formula . We tried to every way to get it down but he ether threw up oat milk when it was more 60% mix or with coconut milk that was toddler safe he just wouldn't drink it . Do what works for your little ones digestive system
I think it’s your choice because almond/oat milk in the UK is heavily processed and a lot of parents don’t want to give that to their babies so some would rather continue with their formula and slowly introduce another milk as baby has already familiarised with formula x