Monitoring Remaining Planned Costs

Hello everyone,

I have a question regarding planned costs for future periods.

Example (please correct my procedure if i do something wrong):

At start: 

Activity with original duration of 2 months costs 1000.

Planned cost by default is Month 1 - 500, Month 2 - 500. Baseline saved.

After month 1. 

Actual cost is 200 only, also Contractor asks for extention, so I extend duration at completion to 3 months. Original remains as 2 months.

I Update progress for past month, then I Save actual performance of 200 in financial period 1. Baseline saved.

After month 2. 

Actual cost is 200, but Contractor promises to finish it on time (end of Month 3).

I Update progress for past month, then I Save actual performance of 200 in financial period 1. However, after updating progress Activity status automatically changes to Completed in Month 2 (as it was initially planned). But I don't want it, besides I want to see planned cost for Month 3, which is now shown as 0.

I think that both issues related to original duration. So I cancel completion status, and extend original duration so it is equal to Duration at completion (not sure of this).

Planned costs now changed: Month 1 - 333, Month 2 - 333, Month 3 - 333. 

But we know that Cumulative Actual cost for Month 1 and 2 is 400, and  at this point by logic in Month 3 Planned cost must be remaining cost =Planned-Actual=1000-400=600. But it is seen from Planned costs that Month 3 is just 333. 

So, what do I do wrong?

1. Can I change original duration? Do I have to do this when I use Update Progress?

2. How to see correct remaining Planned cost for future period? 

3. What technique should I use  in this case - Update Progress or something else?

Thanks in advance!