BGP: Need to advertise public route for test network

I am upgrading equipment for an internet app at a site (Site A) that uses provider independent addresses connected to an ISP, running eBGP. I need to have the servers running concurrently from the same location to test the new machines and code, but I cannot use any of the production address space.

I can subnet out a /28 from a class c provider independent address block in use at a different location (Site B)(different AS), but am not sure how to make it workable. Is it as simple as modifying BGP from losing site (Site B) (VLSM) to drop advertising the removed subnet block and creating BGP entries for that block from the shared (new) location (Site A)?