No, it should be possible to use the same mail. Your dashboard will be switched to the new reseller
No, you can't use an e-mail adress which is assigned to a dashboard again for a different dashboard. That's the problem of OP. If it would work he wouldn't have asked the question.
As I said, it might be possible if the reseller which assigned the e-mail to the dashboard disabled the dashboard, but I am not sure. It's definitely
not possible if one dashboard is still active though.
I had this problem lately when I tried to assign my client a dashboard. I got an error message that the e-mail has already been used, although the subscription of his previous reseller has run out already.