You can copy the sparsebundle file from one drive to another, but I believe you'll have to continue the backup wirelessly (not too sure on the second part of my sentence).
So, if you connect the new drive to the TC, mount the drives in finder and copy over the backup, you should be able to target the new drive with your iMac and continue the backup.
I've copied backups to and from multiple drives, both for installing a larger hard drive into my Time Capsule as well as for copying it over from one Time Capsule to another. They all allowed me to continue my backups without losing previous backups.