The US-UK extradition treaty doesn't need to be renegotiated. It just needs to be enforced. The treaty makes it very clear that Assange cannot be extradited:

extradition shall not be granted if the competent authority of the Requested State determines that the request was politically motivated.

The only question is whether the UK government will breach the treaty in order to please its American allies.

1. https://www.gov.uk/government/publications/extradition-treat...