Swapping Phones while on Next plan

No, it is not possible to move the Next plan. The Next plan is tied to that number but you can use any phone you want on any number. If you want to use a different phone on that number or use that XS Max on a different number, you can. Just move the SIM cards. However, the Next plan for the XS Max stays with whatever line it is on.

Ok thanks for the help, so what I'm getting is that the phone will stay on that line but i can just swap my current sim card into the phone I'm buying and put my family members sim into my XS Max? And even if  upgrade the xs max all i have to do is put her SIM into the upgraded one?

Yes, you can use both phones on any line by swapping SIM cards. 

Thanks so much!!