It is examination that evaluates your awareness, accumulated knowledge and personality that is suitable for the job in question.
In the beginning, you should not think too much about it and should concentrate on exam related requirements.
It is really tricky situation in front of Hindi medium candidates and in past few years, result of Hindi medium candidates has been going down.
Recent developments suggest re-look into the new Preliminary Examination pattern that includes CSAT that has adversely impacted the fortunes of Hindi/vernacular languages candidates. What would be the outcome, only time will tell and till then, you have to keep yourself ready for the existing set-up.
As far as changing medium of writing examination is concerned, it is a tough decision. You are a Hindi medium candidate and preparation and writing examination in English medium may not get you desired results until you work very hard.
As you are to appear in CSE 2015, you have enough time and hopefully something emerge that can uplift the chances of Hindi/vernacular languages candidates; so, in the mean time you should continue with your preparation-plan.
The following article will help you in getting some insights about the medium of examination.