Just to clarify: if you use a backup tool like MyBackup, you have the option to save your backup to your SD card, or you can save it to their servers (on the cloud). In your case, you'll want to back up to the cloud, since you'll be returning the SD card with the phone.
The alternative is to back up to the SD card, copy the relevant folders off the SD card to your PC. Get your new phone, put it back on the SD card, then restore with the backup app. Lots more work, but this way, you control your own data all the time.