用英语怎么说“新合同生效时旧合同就自动失解除法律关系”

The previous contract becomes null and invalid when the new contract comes into effect.

the first-signed contracts wil become null and void after the second-signed contracts come into force.

When the new contract takes effect, the old contract will cancel legal relationship automatically.
呵呵，自己翻译的，不是用所谓的机译。

When the new contract goes into effect,the legal nexus of the former contract shall be withdrawn automatically.
这是最标准的法律英语~