She's not too young at all

But you just need to read the labels carefully, because not all of the Timberwolf formulas are ideal for pups. Their Wild N Natural is best suited to dogs that are 18 months old and upward. And the Black Forest and Lamb/barley may also be formulas to avoid for now (the former being a little low in protein/fat for a baby and the latter a bit high).
All of the remaining formulas though, should be perfectly suitable for both your dogs (bison, elk, ocean blue, southwest chicken).
As for how to switch, the best way is gradually. You're far less likely to have any tummy upsets that way. Start by giving her a mix that is 75% old food and 25% new food. Feed that for a couple of days or so, and if there are no tummy issues, increase the blend to 50/50. Again, a couple of days or so, then up to 25/75 and finally to 100% new food. All going well, that should take about a week to ten days to accomplish. Just note though, that if at any point the pup does get loose stools or a tummy upset, hold off on increasing the amount of new food in the mix until she has adjusted and all signs of upset are gone. Then you progress to the next step.