I have a motobecane phantom cross with about forty miles. I would like a mountain bike with drops. Can I simply switch out some of the main components to a mountain frame (specifically the cranks and rear cassette? I also have a redline monocog. Can I take the wheels off of that and stick the rear cassette on a wheel? Thanks.

Tube diameters on the frame and the wheel size the same?

700c=/= to 26",so mtb conversion needs the right wheel size.

N+1 , buy another bike , of the type you now desire.

You bought an online bike and found it was not every thing you wished..
try a Bike Shop this time,
maybe they will take a consignment sale of the other one.

How lucky are you usually?

Everything on a bicycle works together. You might be able to switch out parts as you mentioned, but I doubt it. The monocog cassette swap is a definite nogo. The crank swap probably won't work either because road bikes usually have a 68mm bottom bracket shell and mountain bikes are 72mm. We haven't mentioned whether the shifters and derailleurs will work, but they're going to be iffy too.

Frankly, if you're feeling lucky, I think you'll be better off buying a lottery ticket and planning on using your winnings to buy the bike that you want.

The monocog as well as other have been bought at lbs. I had ebay $ I needed touse, and neither lbs was he least bit intetested in cyclocross bikes. I am known by face and name at one lbs btw.

Thanks, I suspected as much. Ill just wait as im not keen on spending anymore money on bikes right now. Iwish I could do something with the monocog.