If the bike fits well you could certainly use it like a road bike if you wish. There are threads here dedicated to folks converting mountain bikes to ride exclusively on pavement. A lot has to do with the tires that you install -- smooth tread as opposed to knobbly. But you do need to keep in mind that if the gearing is set up for mountain biking or cruising, you would not have the high-end of dedicated road bikes and so would have a lower top speed.

It is sometimes possible to change wheel size without needing dramatic efforts but only when the change in size is minor. I've only ever personally interchanged wheels between 27 and 700c, which is a very small difference. I've heard that you can sometimes use 27.5" wheels on a bike set up for 26" wheels depending on your fork and brake situation. But going from 26" to 700c or 27" would present some serious issues for the brakes and potential clearance, and would change the handling of the bike.