It’s currently possible update to iOS 7 without having your iPhone’s UDID registered in a developer account.
Do not do this if you want to retain your jailbreak. You will not be able to get it back.
Make sure your device is activated before clicking update.
Make sure your device is backed up.
To perform the update you must get a hold of the iOS 7 ipsw here.
Press and hold the Option key (Shift for Windows) and click Update in iTunes if your device is not in Apple developer programme. If it is choose restore button.
Select the iOS 7 ipsw from the popup window to install.
It’s unclear why this is working. Only iPhone devices registered by developers are supposed to be able to install beta firmware. Perhaps Apple took down this check since its servers are being flooded at the moment.