Hi Paniz, you can pick any smart phone you want on any plan. What is your new plan?
If it's above $35, you can qualify for a tab M which is $300 off the phone however there would be a $5 tab charge every month.
If not, you could pick a tab S ($150 off) for whichever phone you choose.
"...I want to upgrade my plan getting a new smart phone and having the same plan as before..."
Do you want to change your plan or you want to keep current plan?
You can pick any phone you want with a Tab S and keep your current plan.
You don't have to change your plan when you upgrade your phone unless you want to use a Tab M but you have a plan under $30.