The USSR was relatively speaking new. The territories it gained in the West were not those that used to be part of it and because of the feeling that they belonged to their land and not the USSR created many of the issues that led to its downfall. Ukraine for instance was resistant and Stalin had to kill 5 million people and cause a famine to get that area under control. Unless you think Mexico or Canada are going to try and annex part of the US then I think we can say for now its highly improbable.

Also the US and Mexico do change the borders fairly often because of the Rio in Texas. As the river moves so does the border.