I have no experience with overtraining. Based on the forum discussions related to this topic I think it will be challenging for you to regain your fitness in two weeks and be ready for a four-day (?) high altitude climb in South America. Would this be your first climb in this region and at this altitude?
Even though on the first day of your transition you feel strong, I would just recommend being conservative and take it slow. You don’t want to slip back into overtraining and should allow your body to recover.
Check out the forum topics related to Overtraining (under Forum Index on the right). Also check this topic where Scott has provided some great inputs to someone getting back into training after a long break.