Backup and Restore Entire NAS on Bigger HDD

Well, sortta.  You can back up the configuration via the Settings page of the GUI and data via backup jobs.  Some app configurations will be backed up and some apps will require you to separately back up their data, especially if it's in a database or was installed via SSH instead of a ReadyNAS-specific app.  You cannot back up the apps themselves.  Then you swap out the drives and create a new volume.  Be sure to re-install the apps before you restore the backed-up configuration.  Since you aren't destroying the original volume, you can always swap the drives back and get anything that you forgot to back up.  With two volumes, you could replace only one of the drives now and re-use an old one, but that would make recovery from the old one impossible.

FWIW, RAID0 is a very vulnerable configuration,  Any loss on one drive kills the data on both.  You may want to consider setting up the drives as separate volumes.  That means you have to manually distribute content between the two, but it makes recovery from a drive failure easier.  Note that it will still have no redundancy (which really isn't enough to protect your data, but helps) I hope you already have and plan to continue to have a backup unless the NAS is already only backup data.