Adopted dog suddenly misbehaving

Question
Hi there, we recently adopted a 5 year old jack Russell. He's really quite calm and very good natured towards anyone. We've been taking him to training classes, but he has suddenly started misbehaving. =( We've had him for a month and he's never gone upstairs(we'd like to keep him only on the main floor) but just yesterday we found him in the upstairs bathroom. He has also been jumping up on sofas and refusing the walk. He's never had these problems before. When our family goes out, we usually crate him, but sometimes we don't as he seemed to be a very well behaved dog.

Any feedback would be great, my mother at the moment is thinking of surrendering him back to the shelter due to these sudden problems.

Answer
Well you have to make sure he knows you are in charge,now that he is comfortable in his home he may now think he is in charge

As soon as he jumps up on the sofa,take him off and say "off" loud and firm,and when he goes in rooms you don't want him in,take him out of the room and say 'out'

Just keep up with his training as well,as that usually helps alot