8.X migration assistant does not make any changes to 7.x or take down any service .
It is up to customer to tear down the old 7.x or keep it running .
Since vRA 7.6 is out of General support , I would recommend to shutdown because no feature or further release would be coming .
As above mentioned , it is up to user , vRA8.x would not cause any service down time to 7.x
