How Easy Is It To Convert A Property's Classification, And All That Entails?
If I purchase a two-family dwelling in a predominately one-family neighborhood where the one-family properties are selling for much more then the two's, how easy is it to convert the subject property? Also, what are the costs involved? Thanks all! 
Check with the zoning board and the building code dept in your area
I would first confirm the current zoning by consulting the zoning map and ordinance and then by speaking with the zoning officer to find out if the property is non-conforming -- meaning that its use as a 2-family predated the zoning ordinance. If it is non-conforming, the zoning officer can advise you if and how the 2-family use can be "abandoned."
. Discuss this with the zoning officer as well.
If the property is conforming--that is, a true 2-family--a zoning change may be necessary. The difficulty and legal expense vary by area although in this case, you are, in a way, decreasing the density and not increasing it, so perhaps you wouldn't have the same opposition as if you were trying to increase density (or "down zone"
You would also have to take into account the cost of physical changes to the structure. And last but not least, I would think you'd want to consider your investing objective--rent v. sell--to make sure you're following the course that will best achieve that objective.
Thank you greatly for your advice.