If for the guinea pigs is advantageous to have long fur in order for them to survive, than it can be expected that the guinea pigs will have longer and thicker fur.
This will happen because of the evolutionary pressure to survive, and also because of natural selection, where in the mating it will be advantageous to mate with an individual that has longer fur, while the ones with shorter will be rejected, thus they will not transfer their genes.
Apart from growing a long fur, it will for sure become thicker as well, and probably a layer of thicker skin for better isolation from the cold.