I assume the connections are straight SQL*NET. If so, each desktop would have its own tnsnames.ora file. At a minimum, these would have to be updated to be made aware of the new instance. The last couple of places I worked had a PC support team with software that could push a new tnsnames.ora to each desktop without having to physically be at each one. I understand Oracle Names might be able to do this more easily, but I’m not that familiar with it, and Oracle is moving away from it anyway.