Disable iCloud Backup without deleting existing backup?
If you turn off iCloud Backup on your iPhone, the iCloud Backup isn't deleted from your account immediately. 

The Backup does get "abandoned" some time after it stops receiving updates, I think it's like 6 months... but you should call up a AppleCare rep to confirm what time frame it is, I never found out how long, just that it is months afterward the backups stop from that device