Change a Sign-In ID PS3

Hello,

I have both a PS3 and PS4. I purchased PS+ for my son through the PS4 the other day. It was my understanding that PS+ can be used on multiple systems. So, I went to access it on the PS3. I found that the Sign-In ID on the PS3 is tied to an email that is no longer in existence. I went through the process to change the ID to the email associated with my PSN account. It told me that email cannot be used. Why not? 

This smells like a one off and I think it's crappy that I have to seek help in a forum. Where's the customer service, Sony? 

Thanks for your time!