How to convert between Linq expressions with different return types?

You'll need to create a new expression by:

1. Using Expression.Convert over the source expression's body to create the result's body.
2. Using this body and reusing the parameters of the source expression to create the transformed lambda expression with Expression.Lambda.

Try this:

Expression<Func<T, object>> source = ...

var resultBody = Expression.Convert(source.Body, typeof(U));    
var result = Expression.Lambda<Func<T, U>>(resultBody, source.Parameters);