Please note that 'iCloud Drive' in iOS8 is a brand new feature, separate from iCloud on iOS7. If you choose to use iOS8 iCloud Drive you will not be able to access your iOS7 iCloud items from your desktop or iOS8 device. To access iCloud Drive on desktop, you'll need to wait until Apple releases the 'Yosemite' operating system.
It's always good to have several backups of your artwork, even if you're using a cloud storage provider. We recommend making a local backup of all your SketchBook artwork, and creating a brand new iCloud Drive if you choose to update to iOS8.
Here's Apple's support page on how to make a backup. Don't wait, do it now!