well… it depends… If both repositories are part of the same server pool, than guests can be moved between those repos, while being shutdown. You can't live migrate guests between repos on OVM, anyway. If these repos belong to different server pools, than I'd suggest to do that manually, where manually. The amount of work involved depends on how these guests are configured.
You can refer to the below document:
There subtab - Move this VM to a different Repository
For doing via OVM CLI you can refer the below docs:
Hope this helps.