Tough question to answer. The 'usual' and ME recomended method is to use hot water. That said some people wash in warm and some even in cold and have no problems whatsoever. I'd say give it a shot and see what happens. Worse thing that could happen is that the diapers would get stinky from not being cleaned properly and you'd have to go back to hot. You could also alternated between warm and hot washes as a compromise as well.
All that said, some people do wash their covers with their diapers even in hot. It's not the best method to make your covers last but it won't destroy them instantly either.