Were you looking at the website purchase page or the Stardock Store downloads section? If it's showing on the latter, you should be able to just grab it regardless of the email.
If you're having trouble changing the address on your account, you will need to email support@stardock.com and they can get that taken care of.
I tried opening Impulse for the first time in, as my original post said, six months...Could this be related?
No, Impulse is owned and operated by GameStop and is completely separate from our account systems.