So I no longer have access to my original twitch account. Lost the email and don't remember the password.
Is there a way to change the twitch account on an already purchased stream slot or do I need to purchase another?

Thanks in advance!

I'm pretty sure you would be required to use the Add New Stream feature, as technically that would count as a new stream.
But if you want to be 100% sure you could always try PMing a Moderator who manages that section.