Is it possible to have Schengen Type C and Type D visas at the same time? What if I have a Type D visa and due to change in circumstances, I would like to visit the country as a tourist only?
A Type D visa simply is a longer term visa; it removes the 90 day limit from a single stay; and is usually (but not always) given for specific reasons (such as for study) and is normally limited to a year.
Once you are in the country that issued you the Type D visa, you can visit other Schengen countries, but your stay in the other countries is subject to the same 90/180 rule as if you were on a type C visa.
If your circumstances have changed based on which you were granted the Type D visa; you should visit the embassy and ask for an appropriate visa for your intended purpose pf travel.