My contract is ending at the end of January (covering maternity leave). My boss and I agreed that they would create a new position in the office for me so that I can stay and continue working there. This new position created is the only reason why I would stay with the company longer. However, last week one of my colleague gave his notice so now the company is having me train with him to cover for him when he is gone. I'm sensing the company will just move me to his position instead of creating a new position for me like discussed. I do not want to take my colleagues position at all. How do I let my boss know when they bring that up to me? I'm willing to cover until they find someone but I am not interested in staying for that position.
You just have to be honest and tell them this is not what you were expecting based on discussions and you aren't interested.
You have to be careful abbout "covering until they find someone". That can be extended for a long time if you let it happen. Be polite but firm. You want the new position; you could even offer to help them to find someone to replace the other person but make it clear that this is not going to be you.
Anyway that is what I would do.