Just say you want to update (sounds more harmless than "change") your email because you can't access your current one. If they ask for more details, say you changed ISP's, and you lost your first email.
Here's how you recall an account (copy and pasted from one of my other guides):
Recalling an account is the act of calling up Blizzard's Billing & Account Services Department (also known as just "Billing") and saying that you want to update your email. If you can provide the account's first and last name, answer to secret question, and its phone number when you still had it, then they'll be able to set the account's e-mail to your e-mail. Recalling is generally supposed to be used if you get hacked, but it can be used to scam people or to get an account back after a scam.
Essentially, whoever owns the email owns the account, so this is a very important part of scamming.
Here's an example of how the conversation will go when you recall a WoW US account. Note, it may not go exactly like this, but it'll most likely go very similarly (if you are recalling a US account.)
Courtesy of Flawz. This example was written by Flawz and edited slightly by me.
-Blizzard: Hello this is Dave from Blizzard Billing and Account services. Can I have your phone-number first?
-Me: 928-123-4587 (Tell them the number you're calling from, not the number on the account. Also, they may not ask this question.)
-Blizzard: Alright thank you. How can I help you today?
-Me: I'd like to update my WoW account's email. It appears my email account was deactivated when I changed ISP's and I'd like to change the e-mail on my account.
-Blizzard: That shouldn't be a problem. I'm just going to ask you a few things.
-Me: Okay.
-Blizzard: What is your account name?
-Me: <<Tell them the acc name>>
-Blizzard: Alright, what is the registered first and last name on the account? (Note, they may simply say "what is your name." The point is, you're pretending to be that person. If they say "what is your name?", give them the name on the account.)
-Me: <<Tell them the full name on the account you scammed>> (While you do this, you should spell out the name, letter-by-letter.)
-Blizzard: (This next question is not always asked; I'm not exactly sure why. Basically, they'll ask what the current email on the account is. If they do and you know it, give it. If you do not know the answer, say that the email account was deactivated a long time ago and you don't remember it. If they then start asking you more questions, seem very suspicious, or refuse to go further, just hang up, wait a few minutes, and call again.)
What is the current email on the account?
-Me: <<Tell them the email.>>
-Blizzard: Alright, I need to know the secret answer to the secret question. <<Asks you the secret question>>
-Me: <<Tell them the secret answer you scammed>>
-Blizzard: Thank you. That is correct. What do you want the e-mail changed to?
-Me: <<Tell them the e-mail you want it changed to...an e-mail address that you own and use>>
-Blizzard: Done, is there anything else I can help you with today?
-Me: That should be everything. Thank you very much. Bye.
-Blizzard: Good bye.
To sum it up, they'll probably ask for phone number, then the account name, then the first and last name on the account, then possibly the current email, then the answer to the secret question (SQA), then they'll ask what email address you want it changed to.
So, you say you don't know the current email. If you call up and they ask for it, say you don't know it. If they then get suspicious/refuse to continue, hang up, call again, and see if you get someone else.