Gather all evidences you have first like
Smses or anything else documented proof which states that your nonstop is valid till 2020
Trai's notice about long term plans
SMS that indicates your pack changed without consent
Conversation where they lied that your non stop valid only till Feb 2019
Once you have everything draft a mail mentioning wrongful change of pack without consent, explaining everything keep it precise mark every point with the documented evidence send to dish, nodal officer, trai
And if no action is taken inspite of this then contact consumer forum with evidence