Upgrading free Netflix with T-Mobile

I’m currently with Verizon but I’m about to pull the trigger and swap to T-Mobile. I noticed the plan I’m looking at comes with free Netflix and Hulu (both with ads). I’ve read where Netflix can be upgraded to premium for an extra fee (I’m not sure about Hulu). How does that work if I already subscribe to Netflix premium? Will I need to cancel my existing service and basically start over? Or can I keep my current subscription and start receiving a discounted rate because of T-Mobile? And I assume the free Hulu does not include Disney Plus? If not, I wonder how that will work in the future? Because I’ve heard Hulu is going away and getting fully integrated into Disney Plus.