Encrypt existing unencrypted Time Capsule backup
The only solution I can offer is to use Disk Utility to create an encrypted sparsebundle, then copy the contents of the existing backup disk into the sparsebundle. This is essentially what Time Machine does.

Then, select a new backup disk in Time Machine and select the sparsebundle. If all works according to plan, Time Machine will ask for your password and begin using the new sparsebundle.