You can transfer ownership in the G+ Dashboard. I highly recommend having the current agency turn over the email address that's associated with ownership of all of the location listings. With that, you won't actually have to transfer ownership in G+, only IRL with the actual account & password access.
Yes, the previous agency can delete the listings. You would have to reverify all of them if they go this route.
You can also go about getting these via the bulk upload process & having the ownership of these businesses authorize Google to transfer ownership of all 100 listings to what ever email address you choose. This isn't a quick process. Suggestion 1 is the best route.
~Scott Davis, Senior SEO Specialist at Valet Interactive Marketing
