Conversion Helper with support for enumerations and Nullable - The Remix
As a good bit of time has passed, and no additional feedback seems to be incoming, I'm posting this answer so that I can close the loop on this question. 

The only feedback I might give -- and it's a small optimization -- is to move this code:

Type nullableType = Nullable.GetUnderlyingType(originalType)

I think it would be slightly better to put it after the check to see if the value parameter's type equals the generic type parameter's type, as there exists a chance that the method will not need that variable at all if the types are equal.