The primary email address for my Google account is a Gmail address.
How do I change the primary email address on my Google account?
I don't think that you can do that. I would suggest loggin with your other GMail account that you want to make it primary and then add the old one as an additional email address to connect them.
You could also forward all your email from one to another or simply retrieve them via POP, if it is only for your emails.
You need to fill out a form (contact Google) to do this. Here is the form.
A Gmail email address will always be the primary email address of a Google account having Gmail, also an email address that is the primary email address of a Google account can't be used as the primary or alternative email address of a different account.
While it is possible to remove a Gmail address from a Google account that email address will not be possible to be used again.
Considering the above, what you are trying, change the primary email address and keeping the original primary email address, in other words having two Gmail address in one Google account, is not possible.
first you have to add alternative email address to your gmail account then delete your gmail account. Remember if you delete you gmail account all the account accosted with gmail has been deleted. http://support.google.com/accounts/bin/answer.py?hl=en&answer=32046