Enhancement: Reassign Instance to a Different ISO or Template

### The required feature described as a wish

As a User I would like to unassign and reassign an instance to another ISO or template. It's not possible to guarantee a template will always be around and it's a waste of space to keep older templates. We have a need to update templates on a periodic basis.

The issue names no files, tests, or entry points and does not define the reassignment behavior or acceptance criteria. First trace how CloudStack associates an instance with an ISO or template and identify the relevant API and tests. Before implementation, confirm whether both ISO and template reassignment are required and define what “done” means for existing instances.

Written by the indexing model from the issue text.