Usually youd have to retrain as far as I know, especially if you wanted to go in to MH as youd be trained to work with addiction and in forensics and with elderly. There are child and adolescent psychiatric wards so MH nurses also train in that on placements but as above said, you may be trained in that also.
Im not 100% on which skills are taught to all pathways but Im pretty sure you would need to retrain, at least 2nd and 3rd year.