So, there are two specific rulings here, for the sake of clarity:
- When you have a talent that lets you replace which skill you roll for a test, use the new skill’s normal TN and Focus. This is for the sake of simplicity.
- When you have a talent which lets you replace which skill you roll for a test, that test still counts as what it was originally: if you’re using Persuade on a Command test, it’s still a Command test, you just get to use your Persuade skill instead. Again, this is for simplicity - I’ve seen people try to justify using one skill replacement talent to set up a second skill replacement talent so they could get all sorts of sneaky extra bonuses.