Reset iCloud account from iPhone

Recently i've been trying to upgrade my phone to my BF's old iPhone 4s 16 GB.

i created a backup and restored it into the 4s. All good except one thing: iCloud.

At the same time i was doing this i changed my main email for my iCloud account. The 4s however had the iCloud account set with the old email (which i have no longer access to). I've tried deleting the account from the 4s, and it won't work as it requires the password for the iCloud account with the old email (same account technically but iCloud treats it as a different one apparently).

Now in the old phone the iPhone 4 i was able to delete the iCloud account, with the though of creating a new backup and restoring it into the 4s, however the 4s won't let me delete it since the iCloud account it is set to has since changed emails.

Is there anything i can do? i have access to the iPhone (the 4s), is there anyway i can delete everything? when i try it won't let me as i can't authenticate with the iCloud account it has set to.

Thanks a lot for your help.