The transponder is mapped to a "channel" so they could switch a feed and just remap it without anyone knowing.Even if they do have a very brief period of overlap they must have a way of signaling receivers to switch to the new satellite/transponder.
Commercial receivers in headends are set on the same channel 24x7, they'd be having problems every time a channel moved otherwise.
The one time I noticed it was when our cable provider (Cox) forgot to change the transponder for YES and a splash message appeared on the channel informing the headend of the change that the current transponder was not in use anymore.