TM will not move your old snapshots to a new drive, although I'm pretty sure that if you first used Disk Utility's Restore tab to clone your old backup drive to the new one (after reformatting it properly), it would see the old snapshots properly.
But there's a bigger problem. You are trying to use TM as an archiving tool, but that's really not what it's designed for. TM will expire old snapshots as it sees fit, so if you want reliable retention of old files it's not the right tool for the job. Unfortunately, I don't have a good suggestion for a better tool, but I would strongly recommend looking for something more appropriate.
That said, switching TM to a new drive (and removing the old one from its backup target list) would be a semi-reasonable way to retain all of the old snapshots (and having multiple redundant backups is a good idea). Doing that and switching to a new archiving tool would also be reasonable (since you probably won't be able to migrate the old TM snapshots to any net tool you select).
Can I copy those .sparsebundle files to an external hard disk and expect them to work as Time Machine backup resources again?
Almost certainly yes.
Modes and ownership
Ideally, use chmod(1) and chown(8) to have the mode and ownership, of the copy of the disk image, consistent with how Mountain Lion normally creates a local disk image for Time Machine.
Example
GPES3E-gjp4-1:~ gjp22$ sudo ls -al /Volumes/tall/com.apple.backupd/GPES3E-gjp4-1.sparsebundle/
Password:
total 16672
drwx------@ 3 root staff 10 7 Jun 19:07 .
drwxr-xr-x 8 gjp22 staff 10 6 Jun 18:57 ..
-rw-r--r-- 1 root staff 500 7 Jun 18:03 Info.bckup
-rw-r--r-- 1 root staff 500 7 Jun 18:03 Info.plist
drwx------ 2 root staff 78855 7 Jun 19:07 bands
-rw-r--r-- 1 root staff 444 7 Jun 18:02 com.apple.TimeMachine.MachineID.bckup
-rw-r--r-- 1 root staff 444 7 Jun 18:02 com.apple.TimeMachine.MachineID.plist
-rw-r--r-- 1 root staff 1473 6 Jun 21:28 com.apple.TimeMachine.Results.plist
-rw-r--r-- 1 root staff 10698 6 Jun 22:24 com.apple.TimeMachine.SnapshotHistory.plist
-rwx------ 1 root staff 0 19 Mar 19:36 token
– and for each band, you should have:
-rw------- 1 root staff
Attaching the copy of the disk image
With Terminal, for the example above:
sudo hdiutil attach -readonly /Volumes/tall/com.apple.backupd/GPES3E-gjp4-1.sparsebundle/
If you use Terminal from within Recovery OS: you'll probably not need the sudo prefix to the command.
Additional detail
… whether I can still restore …
You require:
- a copy of a Time Machine destination, a copy that will be good for restoration
- not an additional Time Machine destination.
Assumption
You wish the copy to be usable, for restoration only, in the event of loss or failure of the Time Capsule.
If the disk is attached from a local image where modes and ownership vary from the norm for a local image:
- OS X or Recovery OS might use that disk without difficulty; I can't be certain.
Thoughts
If you're preparing for the possibility of loss or failure of a Time Capsule – and restoration from a copy of what was there – then think a step further, to backing up without the original Time Capsule. (If you wish the copy to be writeable by Time Machine, that could be a separate question.)
All things considered
It's probably quicker and simpler to use the external hard disk, or a part, as a Time Machine destination.
Best Answer
Take a look at Crash Plan. It allows you to backup to a disk, machine on the network, or the cloud. It has options for how long to retain items and what to ignore.