Why am I unable to change my email address to one that was used by an account I deleted?

That's right, I'm outing myself. I the username change advocate had actually created this account to replace another with a username I didn't want anymore. The difference is that I'd barely used that account, I hadn't established myself with it and had nothing to lose.

Anyway, that is beside the point.

I'm quite certain I deleted that original account when I created this one but I'm not able to convert the email address it used to this account as it tells me it's already registered to an account.

I'd let this go for all these years, so why do I care now?

Well, the email address in question is the one I actively use for communities such as this so it's always been a preference but am also feeling a bit weird since I had to use it when I emailed FP yesterday to see if they take username change requests.

So I'm figuring it's time to get this figured out.

Any thoughts?