请问it would be possible和it is possible 有什么区别?

请问it would be possible和it is possible 有什么区别?
读到一句话
He said that it would be possible to build a platform in the ctre of the Channel
请问如果改成
He said that it was possible to build a platform in the ctre of the Channel
有什么区别?或者错改了就错了?
it would be possible和it is possible 到底有什么区别?
谢谢