[semimojo]

...He [POTUS]shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur;

Yes, Congress can amend it as much as they want

Which is why trade agreements are not implemented as treaties but as congressional-executive agreements.

The time for congressional input on the negotiation is before it's concluded, If Congress doesn't like the way the President acted on their input they can choose not to approve the agreement.

[Erik Latranyi]

Which is why trade agreements are not implemented as treaties but as congressional-executive agreements.

Please cite the constitutional authority to create congressional-executive agreements.