Welcome! I think what most people do about losing the DF is to move it forward gradually until it is meeting the BT feed, that said, if she is sleeping that well you may be able to just drop it cold turkey.
At 12m, your supply is well established, so is unlikely to be affected by stopping the DF now.
After 12m, there's not such a focus on milk for nutrition as there has been in infancy, so if you are concerned about her being hungry, you could offer more food. You're BF more times during the day than a lot of people at that stage (we were probably on 4 feeds per day, no DF at 12m, dropping down to 3 feeds not long after).
