You can use a different e-mail or close your prepaid account online or by calling Customer Care.
There's no need to get a new number with a new SIM card / plan.
You could just contact Customer Care to move your existing number [and balance] to the new SIM. You can even keep the plan you had (if it suits you), or you can move to the new plan if it's better.
(unless you've already activated a new number on the new SIM -- then it's already in use and you can't activate it on a different number)